#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i,n) for(int i=0;i<n;i++)
#define int long long

signed main(void) {
  double a,b,c;
  cin>>a>>b>>c;
  double s=(a+b+c)/2;
  cout<<setprecision(18)<<sqrt(s*(s-a)*(s-b)*(s-c))*.25<<endl;
}