/*
  tester : atetubou's code
*/
#include <bits/stdc++.h>

using namespace std;

int main(int argc, char *argv[])
{
  int c,r1,r2;
  cin >> c >> r1 >> r2;
  long double rc = abs(r1-r2)/2.;
  long double rd = (r1+r2)/2.;
  long double m1 = -1;
  long double pi = acos(m1);

  printf("%.100Lf\n", rc*rc*pi*2*pi*rd*c);
  return 0;
}