#include <iostream>
#include <vector>
#include <cmath>
using namespace std;
using ll = long long;
#define rep(i,n) for(int i=0; i<(n); i++)

int main(){
  double p = 0.25;
  double a,b,c; scanf("%lf%lf%lf",&a,&b,&c);
  double s = (a+b+c)/2;
  double S = sqrt(s*(s-a)*(s-b)*(s-c));
  double ans = S * p;
  printf("%.10f\n",ans);
  return 0;
}