#include <bits/stdc++.h>

using namespace std;

int main() {
    double a, b, c, p;
    cin >> a >> b >> c;
    //scanf("%d %d %d", &a, &b, &c);

    if (a + b <= c || a + c <= b || b + c <= a) {
        printf("输入的边长不合法！");
        return 0;
    }
    p = 1.0 / 2 * (a + b + c);
    printf("%.1lf", sqrt(p * (p - a) * (p - b) * (p - c)));
    return 0;
}