sqrt();

#include <bits/stdc++.h>
using namespace std;
double triangle(double a, double b, double c){
    double p = (a + b + c) / 2;
    double s = sqrt(p * (p - a) * (p - b) * (p - c));
    return s;
}
int main(){
    double a, b, c;
    cin >> a >> b >> c;
    printf("%.2lf", triangle(a, b, c));
    return 0;
}