#include <iostream>
#include <cmath>
int main() {
double a, b, c, s;
std::cin >> a >> b >> c;
if (a + b > c && a + c > b && b + c > a) {
s = (a + b + c) / 2;
std::cout << sqrt(s*(s-a)*(s-b)*(s-c)) << std::endl;
}
else {
std::cout << "不能构成三角形" << std::endl;
}
return 0;
}