import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Random;
import java.util.StringTokenizer;

public class Coasters {

    public static void main(String[] args) throws IOException {
        // -1 0 2 -1 -2 3 2

        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer tk = new StringTokenizer(br.readLine());
        int x = Integer.parseInt(tk.nextToken());
        int y = Integer.parseInt(tk.nextToken());
        int r = Integer.parseInt(tk.nextToken());
        int ax = Integer.parseInt(tk.nextToken());
        int ay = Integer.parseInt(tk.nextToken());
        int bx = Integer.parseInt(tk.nextToken());
        int by = Integer.parseInt(tk.nextToken());

        Random random = new Random(System.currentTimeMillis());

        int hits = 0;
        int miss = 0;

        double abx = (bx - ax);
        double aby = (by - ay);
        double rr = r * r;

        for (int i = 0; i < 100000000; i++) {
            double px = random.nextDouble() * abx + ax;
            double py = random.nextDouble() * aby + ay;

            double dx = px - x;
            double dy = py - y;

            if (dx * dx + dy * dy < rr) {
                hits++;
            }
            else {
                miss++;
            }
        }

        double recArea = Math.abs(abx * aby);
        double area = recArea / (hits + miss) * hits;

        System.out.println(area);
    }
}