#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, srt, end) for (long long i = (srt); i < (long long)(end); i++)

void solve() {
    long double a, b, c;
    cin >> a >> b >> c;
    long double s = (a + b + c) / 2;
    long double area = sqrt(s * (s-a) * (s-b) * (s-c));
    double ans = area / 4.;
    printf("%.20lf\n", ans);
}

int main() {
    solve();
    return 0;
}