/**
 *   @FileName	a.cpp
 *   @Author	kanpurin
 *   @Created	2021.04.23 22:13:59
**/

#include "bits/stdc++.h" 
using namespace std; 
typedef long long ll;

int main() {
    int a,b,c;cin >> a >> b >> c;
    double s = (a+b+c)*1.0/2;
    printf("%.10f\n",sqrt(s*(s-a)*(s-b)*(s-c))/4);
    return 0;
}