#include <iostream>
#include <cmath>

using namespace std;
typedef long long int lli;

int main()
{
  lli c, ri, ro;
  cin >> c >> ri >> ro;

  const double r = (ro - ri) / 2.0;
  const double rm = ( ro + ri ) / 2.0;
  
  cout << c * ( M_PI * r * r ) * ( 2 * M_PI * rm ) << endl;
  
}