#include <iomanip>
#include <cmath>
#include <iostream>
using namespace std;
int main() {
    int a, b, c; cin >> a >> b >> c;
    double s = (a + b + c) / 2.0;
    cout << setprecision(10) << sqrt(s * (s - a) * (s - b) * (s - c)) / 4 << endl;
}