#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef long double lf;
typedef pair <int, int> pii;
typedef pair <ll, ll> pll;

#define TRACE(x) cerr << #x << ' ' << x << endl
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define REP(i, n) FOR(i, 0, n)
#define _ << ' ' <<

#define x first
#define y second

#define fi first
#define sec second
#define se second
#define mp make_pair
#define pb push_back

const int MAXN = 410;

int n;

typedef pair <lf, lf> pt;
pt P[MAXN];
pt A, B;


lf dist(pt a, pt b) {
  return sqrt((a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y));
}

lf distZ(pt a, pt b, lf va, lf vb) {
  return sqrt((a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y) + (va - vb)*(va-vb));
}

bool is_in (pt t) {
  int cnt = 0;
  REP(i, n) {
    int j = (i + 1) % n;
    if (abs(P[i].y - P[j].y) < 1e-12) continue;
    if (t.y <= max(P[i].y, P[j].y) && t.y >= min(P[i].y, P[j].y) && P[i].x <= t.x) cnt++;
  }
  return (cnt % 2);
}


vector <pair <lf, int> > v;
lf f(pt p) {
  lf ret = 1e18;
  if (is_in(p) == 0) return 0;
  REP(i, n) {
    int j = (i + 1) % n;
    lf dist1 = dist(P[i], p);
    lf dist2 = dist(P[j], p);
    pt p2;
    if (P[i].x == P[j].x) {
      p2 = {P[i].x, p.y};
    } else {
      p2 = {p.x, P[i].y};
    }
    lf dist3 = dist(p, p2);
    if (dist1 <= dist2 && dist1 <= dist3) {
      ret = min(ret, max(abs(P[i].x-p.x), abs(P[i].y-p.y)));
    } else if (dist2 <= dist1 && dist2 <= dist3) {
      ret = min(ret, max(abs(P[j].x-p.x), abs(P[j].y-p.y)));
    } else {
      ret = min(ret, max(abs(p2.x-p.x), abs(p2.y-p.y)));
    }
  }
  return ret;
}

int REPE = 0;


lf solve() {
  pt tmp = A;
  lf deltax = 0.5, deltay = 0;
  if (A.x == B.x) {
    deltax = 0;
    deltay = 0.5;
    if (A.y > B.y) deltay = -deltay;
  } else {
    if (A.x > B.x) deltax = -deltax;
    deltay = deltax * (A.y - B.y) / (A.x - B.x);
  }

  lf sol = 0.0;
  lf last = f(tmp);
  REP(tt, REPE) {
    pt tmp_next = {tmp.x + deltax, tmp.y + deltay};
    lf h1, h2;
    h1 = last;
    h2 = f(tmp_next);
    sol += distZ(tmp, tmp_next, h1, h2);
    tmp = tmp_next;
    last = h2;
  }
  return sol;
}


int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n;
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  if (a == c) {
    REPE = abs(b - d) * 2;
  } else {
    REPE = abs(a - c) * 2;
  }

  A = {a, b};
  B = {c, d};
  
  REP(i, n) {
    cin >> P[i].fi >> P[i].sec;
  }

  cout << fixed << setprecision(10) << solve() << endl;
  return 0;
}