#include<bits/stdc++.h> 
using namespace std;
const int mod=1e9+7;
int main(){
  double a,b,c;
  cin>>a>>b>>c;
  cout<<setprecision(10)<<sqrt((a+b+c)/2*(a+b-c)/2*(a-b+c)/2*(-a+b+c)/2)/4<<'\n';
}