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

int main() {
  double c, rin, rout;
  cin >> c >> rin >> rout;
  cout << fixed << setprecision(10) << c * M_PI * M_PI * (rin + rout) * (rin - rout) * (rin - rout) / 4.0 << endl;
  return 0;
}