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

typedef long long LL;
typedef double LD;
typedef pair < int, int > PII;
typedef pair < LL, LL > PLL;

#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()

LD a, b, r, a1, a2, b1, b2;

LD steps = 1e8;

LD calc(LD x)
{
	LD val = r * r - (x - a) * (x - a);
	if(val <= 0) return 0;
	val = sqrt(val);
	LD y1 = -val + b, y2 = val + b;
	y1 = max(y1, b1);
	y2 = min(y2, b2);
	return y2 - y1;
}

int main()
{
	ios_base::sync_with_stdio(0); cin.tie(0);

	cin >> a >> b >> r >> a1 >> b1 >> a2 >> b2;
	if(a1 > a2)
		swap(a1, a2);
	if(b1 > b2)
		swap(b1, b2);
	
	LD c = (a2 - a1) / steps;
	
	LD last = calc(a1);
	LD ans = 0;
	
	for(LD x = a1 + c; x <= a2; x += c)
	{
		LD tmp = calc(x);
		ans += (tmp + last) * c / 2.0;
		last = tmp;
	}
	cout << fixed << setprecision(4) << ans << "\n";
	
	return 0;
}