include <bits/stdc++.h>

using namespace std;

int main() {

int a, b, c;
double m, n;
cin >> a >> b >> c;
if (a + b > c && a + c > b && b + c > a) {
	n = 1.0 * (a + b + c) / 2;
	m = sqrt(n * (n - a) * (n - b) * (n - c));
	printf("%.2lf", m);
} else {
	cout << "no";
}
return 0;

}