include

include<math.h>

include

using namespace std; int main(){

double a, b, c;
cin >> a >> b >> c;
double s, d;
if(a + b > c && a - b < c){
	d = (a + b + c)/ 2;
    s = sqrt(d*(d-a)*(d-b)*(d-c));
    cout << fixed << setprecision(2);
    cout << s;
}else cout << "no";

}