#include <iostream>
#include <iomanip>
#include <cmath>

double heron(double a, double b, double c) {
    double s = (a + b + c) / 2.0;
    return std::sqrt(s * (s - a) * (s - b) * (s - c));
}

int main() {
    double a, b, c;
    std::cin >> a >> b >> c;
    std::cout << std::setprecision(10) << heron(a, b, c) / 4.0 << std::endl;
    return 0;
}