#include <bits/stdc++.h>

using namespace std;

double segint(double x, double r, double uy, double dy) {
	double up = sqrt(r * r - x * x);
	double bot = -up;

	double ups = min(up, uy);
	double bots = max(bot, dy);
	if (bots < ups) {
		return ups - bots;
	}
	else return 0;
}

int main(void) {
	// LONG DOUBLE?!
	double x, y, r, ax, ay, bx, by;
	cin >> x >> y >> r >> ax >> ay >> bx >> by;
	ax -= x;
	bx -= x;
	ay -= y;
	by -= y;
		double lx = min(ax, bx);
		double rx = max(ax, bx);
		double uy = max(ay, by);
		double dy = min(ay, by);
		double steps = 10000000;

		double sum = 0;
		lx = max(lx, -r);
		rx = min(rx, r);

		double stepsize = (rx - lx)/steps;

		for (int i = 0; i < steps; i++) {
			double x = lx + i * stepsize;
			double a = segint(x, r, uy, dy);
			double b = segint(x + stepsize, r, uy, dy);
		sum += (a+b)/2 * stepsize;
	}
	printf("%0.10f\n", sum);
			
}