#include <bits/stdc++.h>
#define rep(i, l, r) for (int i = l; i <= r; i++)
using namespace std;
typedef long long ll;
struct point {
    double x, y;
    point() {}
    point(double _x, double _y) : x(_x), y(_y) {}
    point operator-(const point &rhs) const {
        return point(x - rhs.x, y - rhs.y);
    }
    double operator*(const point &rhs) const { return x * rhs.x + y * rhs.y; }
    double norm() { return x * x + y * y; }
    void read() { scanf("%lf%lf", &x, &y); }
    void print() { printf("(%lf,%lf) ", x, y); }
};
double cross(point a, point b) { return a.x * b.y - a.y * b.x; }
double cosa(double la, double lb, double lc) {
    return (lb * lb + lc * lc - la * la) / (2 * lb * lc);
}

double get(point a, point b, int r) {
    if (cross(a, b) == 0) return 0;
    // a.print();
    // b.print();
    // printf("\n");
    point c = b - a;
    double la = pow(a.norm(), 0.5), lb = pow(b.norm(), 0.5),
           lc = pow(c.norm(), 0.5);
    double ca = cosa(la, lb, lc), cb = cosa(lb, lc, la), cc = cosa(lc, la, lb);
    double sa = pow(1 - ca * ca, 0.5), sb = pow(1 - cb * cb, 0.5);
    double s2a = 2 * sa * ca, s2b = 2 * sb * cb;
    double C = acos(cc);
    // printf("%lf %lf %lf \n", la, lb, lc);
    // printf("%lf %lf %lf \n", ca, cb, cc);
    // printf("%lf %lf\n", sa, sb);
    // printf("%lf %lf\n", s2a, s2b);
    double ret = r * r / (8 * b.norm() * sa * sa) * r * r * (C * 2 + s2a + s2b);
    // printf("%lf\n", ret);
    if (cross(a, b) < 0) ret = -ret;
    return ret;
}
const int N = 105;
int n;
point a[N];

int main() {
    point base;
    base.read();
    double r;
    scanf("%lf", &r);
    scanf("%d", &n);
    rep(i, 0, n - 1) {
        a[i].read();
        a[i] = a[i] - base;
    }
    a[n] = a[0];
    double ans = 0;
    rep(i, 0, n - 1) {
        double tmp = get(a[i], a[i + 1], r);
        ans -= tmp;
        // printf("%.8lf\n", tmp);
    }
    printf("%.8lf\n", ans);
    return 0;
}