#ifndef LOCAL
//~ #pragma GCC optimize("O3")
#endif
#include <bits/stdc++.h>
using namespace std;
#define sim template <class c
#define mor > muu & operator<<(
#define ris return *this
#define R22(r) sim> typename enable_if<1 r sizeof(dud<c>(0)),muu&>::type operator<<(c g) {
sim> struct rge {c b, e;};
sim> rge<c> range(c i, c j) {return rge<c>{i, j};}
sim> auto dud(c*r)->decltype(cerr << *r);
sim> char dud(...);
struct muu {
	#ifdef LOCAL
	stringstream a;
	~muu() {cerr << a.str() << endl;}
	R22(<) a << boolalpha << g; ris;}
	R22(==) ris << range(begin(g), end(g));}
	sim mor rge<c> u) {
		a << "[";
		for (c i = u.b; i != u.e; ++i) 
			*this << ", " + 2 * (i == u.b) << *i;
		ris << "]";
	}
	sim, class b mor pair <b,c> r){ris << "(" << r.first << ", " << r.second << ")";}
	#else
	sim mor const c&){ris;}
	#endif
};
#define debug muu() << __FUNCTION__ << "#" << __LINE__ << ": "
#define imie(r...) "[" #r ": " << (r) << "] "
#define range(a, b) "[[" #a ", " #b "): " << range(a, b) << "] "
#define arr(a, i) "[" #a imie(i) ": " << a[i] << "] "
using ll = long long;
using ld = long double;
using pii = pair <int, int>;
const int nax = 410;
int n;
#define x first
#define y second
pii p[nax];
struct L {
	ld a, b, c;
};
muu & operator<<(muu &d, L x) {return d << "[" << x.a << ", " << x.b << ", " << x.c << "]";}
ld vec(pii a, pii b) {
	return a.first * (ld) b.second - a.second * (ld) b.first;
}
L make_line(pii a, pii b) {
	return L{a.y - b.y, b.x - a.x, vec(b, a)};
}
const ld eps = 1e-9;
using P = pair <ld, ld>;
vector <P> inter(L k, L l) {
	#define Q(i, j) ((ld)k.i * l.j - (ld)k.j * l.i)
	ld det = Q(a, b);
	if (abs(det) < 1e-10)
		return {};
	vector <P> ans = {{Q(c, b) / det, Q(a, c) / det}};
	debug << imie(k) imie(l) imie(ans);
	return ans;
}
bool in(P o) {
	int above = 0;
	for (int i = 0; i < n; ++i) {
		int i1 = (i + 1) % n;
		if (p[i].x == p[i1].x && p[i].x > o.x + eps) {
			//~ debug << arr(p, i) arr(p, i1);
			if (p[i].y > o.y) {
				//~ debug << arr(p, i) "above++";
				above++;
			}
			if (p[i1].y > o.y) {
				above--;
				//~ debug << arr(p, i1) "above--";
			}
		}
	}
	//~ debug << imie(o) imie(above);
	//~ assert(above == 0 || above == -1);
	return above != 0;
}
bool in(ld x1, ld x2, ld y1, ld y2) {
	for (int i = 0; i < n; ++i) {
		if (p[i].x >= x1 - eps && p[i].x - eps <= x2 && p[i].y >= y1 - eps && p[i].y - eps <= y2) {
			return false;
		}
		int i1 = (i + 1) % n;
		if (p[i].x >= x1 - eps && p[i].x - eps <= x2 && p[i1].x - eps <= x2 && p[i1].x >= x1 - eps) {
			if (p[i].y < y2 && p[i1].y > y1) return false;
			if (p[i1].y < y2 && p[i].y > y1) return false;
		}
		if (p[i].y - eps <= y2 && p[i].y >= y1 - eps && p[i1].y - eps <= y2 && p[i1].y >= y1 - eps) {
			if (p[i].x - eps < x2 && p[i1].x > x1 - eps) return false;
			if (p[i1].x - eps < x2 && p[i].x > x1 - eps) return false;
		}
	}
	return true;
}
ld distm(P a, P b) {
	return max(abs(a.x - b.x), abs(a.y - b.y));
}
pii make_r(int a, int b) {
	return {min(a, b), max(a, b)};
}
bool in(ld x, pii r) {
	return x >= r.first - eps && x <= r.second + eps;
}
ld alt(P x) {
	if (!in(x)) return 0;
	ld ans = 1e9;
	for (int i = 0 ; i < n; ++i) {
		int i2 = (i + 1) % n;
		ans = min(ans, distm(x, p[i]));
		pii xr = make_r(p[i].x, p[i2].x);
		pii yr = make_r(p[i].y, p[i2].y);
		if (in(x.x, xr))
			ans = min(ans, abs(x.y - p[i].y));
		if (in(x.y, yr))
			ans = min(ans, abs(x.x - p[i].x));
	}
	#ifdef LOCAL
	ld low = 0, high = 1e5;
	for (int i = 0; i < 40; ++i) {
		ld med = (low + high) / 2;
		if (in(x.first - med, x.first + med, x.second - med, x.second + med))
			low = med;
		else
			high = med;
	}
	debug << imie(x) imie(low) imie(ans);
	assert(abs(low - ans) < 1e-5);
	#endif
	return ans;
}
ld dist(P a, P b) {
	ld v = (a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y);
	ld h = alt(a) - alt(b);
	ld r = sqrt(v + h * h);
	debug << imie(a) imie(b) imie(h) imie(r);
	return r;
}
pii operator+(pii a, pii b) {
	return {a.first + b.first, a.second + b.second};
}
ld val(pii me, pii they, P wh) {
	int ax = (me.x - they.x);
	int ay = (me.y - they.y);
	return wh.first * (ax + 1) + wh.second * ay;
}
P mid(P a, P b) {
	return {(a.first + b.first) / 2, (a.second + b.second) / 2};
}
int main() {
	scanf("%d", &n);
	pii me, they;
	scanf("%d%d%d%d", &me.first, &me.second, &they.first, &they.second);
	for (int i = 0; i < n; ++i)
		scanf("%d%d", &p[i].first, &p[i].second);
	L path = make_line(me, they);
	vector <pair <P, bool> > special;
	for (int i = 0; i < n; ++i) {
		vector <pii> dx = {{0, 1}, {1, 1}, {1, 0}, {1, -1}};
		for (pii d : dx) {
			pii pp = p[i];
			pii p2 = pp + d;
			L sp = make_line(pp, p2);
			auto r = inter(sp, path);
			for (auto rr : r) special.emplace_back(rr, false);
		}
	}
	set <int> xes, yes;
	for (int i = 0; i < n; ++i) {
		for (int j = i + 1; j < n; ++j) {
			xes.insert(p[i].x + p[j].x);
			yes.insert(p[i].y + p[j].y);
		}
	}
	for (int x : xes) {
		L li{1, 0, x * (ld) 0.5};
		vector <P> is = inter(li, path);
		for (P p : is) special.emplace_back(p, false);
	}
	for (int y : yes) {
		L li{0, 1, y * (ld) 0.5};
		vector <P> is = inter(li, path);
		for (P p : is) special.emplace_back(p, false);
	}
	special.emplace_back(P(me.x, me.y), true);
	special.emplace_back(P(they.x, they.y), true);
	using T = pair <P, bool>;
	sort(special.begin(), special.end(), [me, they](T a, T b){return val(me, they, a.first) < val(me, they, b.first);});
	debug << imie(special);
	int seen = 0;
	int len = special.size();
	for (int i = 0; i < len - 1; ++i) {
		P f = mid(special[i].first, special[i + 1].first);
		special.emplace_back(f, false);
	}
	sort(special.begin(), special.end(), [me, they](T a, T b){return val(me, they, a.first) < val(me, they, b.first);});
	len = special.size();
	ld ans = 0;
	for (int i = 0; i < len - 1; ++i) {
		seen += special[i].second ? 1 : 0;
		if (seen == 1) {
			ans += dist(special[i].first, special[i + 1].first);
		}
	}
	printf("%.10Lf\n", ans);
	debug << imie(in(P(1, 1)));
}