#include <bits/stdc++.h>
using namespace std;

int main() {
    int a,b,c;
    cin >> a >> b >> c;
    double s = (double)(a+b+c)/2;
    double ans = sqrt(s*(s-a)*(s-b)*(s-c));
    ans /= 4;
    cout << fixed << setprecision(18) << ans << endl;
}