include

include

using namespace std;

int main() {

double a, b, c;
cin >> a >> b >> c;
if (a + b > c && c + b > a && c + a > b) {
	double p = (a + b + c) / 2;
	double k = sqrt(p * (p - a) * (p - b) * (p - c));
	printf("%.2f", k);
} else {
	printf("no");
}

return 0;

}