#include <iostream>
#include <cmath>
using namespace std;

double f(double a, double b, double c) {
    double s = (a + b + c) / 2;
    return sqrt(s * (s - a) * (s - b) * (s - c));
}

int main() {
    cout.flags(ios::flxed);
    cout.precision(4);

    int a, b, c;
    cin >> a >> b >> c;
    cout << f(a, b, c) << endl;
    return 0;
}