#include <bits/stdc++.h>

using namespace std;
using ll = long long;

int main(){

    long double a, b, c, s;
    cin >> a >> b >> c;
    s = (a+b+c)/2;
    cout << setprecision(18) << sqrt(s*(s-a)*(s-b)*(s-c))/4 << endl;

    return 0;
}