#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef long double LD;
#define int LL
#define double LD
const int N = 1e5 + 5;
const int INF = 0x3f3f3f3f;
const double PI = acosl(-1);
const double eps = 1e-8;

struct Point {
  double x, y;
  Point(double x = 0, double y = 0) : x(x), y(y) {}
};
typedef Point Vector;
Point operator+(Point a, Point b) { return Point(a.x + b.x, a.y + b.y); }
Point operator-(Point a, Point b) { return Point(a.x - b.x, a.y - b.y); }
Point operator*(Point a, double p) { return Point(a.x * p, a.y * p); }
Point operator/(Point a, double p) { return Point(a.x / p, a.y / p); }
bool operator<(const Point &a, const Point &b) { return a.x < b.x || (a.x == b.x && a.y < b.y); }
int sgn(double x)
{
    if (fabs(x) < eps) return 0;
    if (x < 0) return -1;
    return 1;
}
bool operator==(const Point &a, const Point &b) { return !sgn(a.x - b.x) && !sgn(a.y - b.y); }
double dis(Point a, Point b)
{
    return sqrtl((a.x - b.x) * (a.x - b.x) * 1.L + (a.y - b.y) * (a.y - b.y));
}
double Cross(Point a, Point b) { return a.x * b.y - a.y * b.x; }
double Dot(Point a, Point b) { return a.x * b.x + a.y * b.y; }
double Length(Vector a) { return sqrtl(Dot(a, a)); }

double calc(Vector a, Vector b, double r)
{
    if (Cross(a, b) == 0) return 0;
    auto Yx = [](double a, double b, double c)->double {
      double t = (a * a + b * b - c * c) / (2.L * a * b);
      return t;
    };
//    cout<<Yx(1,2,sqrtl(3))<<endl;
    Vector c = a - b;
//    cout<<c.x<<" "<<c.y<<endl;
    double da = Length(a), db = Length(b), dc = Length(c);
//    cout<<da<<" "<<db<<" "<<dc<<endl;
    double cosa = Yx(db, dc, da), cosb = Yx(da, dc, db), cosc = Yx(da, db, dc);
//    cout<<cosa<<" "<<cosb<<" "<<cosc<<endl;
    double sina = sqrtl(1 - cosa * cosa), sinb = sqrtl(1 - cosb * cosb), sinc = sqrtl(1 - cosc * cosc);
    double sin2a = 2 * sina * cosa, sin2b = 2 * sinb * cosb;
    double C = acos(cosc);
    double res = r * r / (8 * db * db * sina * sina) * r * r * (2 * C + sin2a + sin2b);
    if (Cross(a, b) < 0) res = -res;
    return res;
}
Point a[N];
void solve()
{
    Point p, c;
    double r;
    cin >> p.x >> p.y >> r;
    int n;
    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> a[i].x >> a[i].y;
        a[i] = a[i] - p;
//        cout<<a[i].x<<" "<<a[i].y<<endl;
    }
    double ans = 0;
    for (int i = 0; i < n; i++) {
        double tmp = calc(a[i], a[(i + 1) % n], r);
        ans -= tmp;
//        cout<<tmp<<endl;
    }
//    cout << ans << endl;
    cout << fabs(ans) << endl;
}

signed main()
{
    cin.tie(nullptr)->ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    int _ = 1;
    while (_--) {
        solve();
    }
    return 0;
}