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

int main() {
    double a, b, c;
    cin >> a >> b >> c;

    double s = (a + b + c) / 2;

    double r = sqrt(s * (s - a) * (s - b) * (s - c));

    cout << fixed << setprecision(10) << r / 4 << '\n';

    return 0;
}