#include <iostream>
#include <cmath>


using namespace std;

int main()
{
    double a, b, c, S, s;
    cout << "Zadejte delky stran trojuhelnika a, b, c:" << endl;
    cin >> a >> b >> c;
    if((a > 0) && (b > 0) && (c > 0)){
    s = (a + b + c)/2;
    S = sqrt(s * (s - a) * (s - b) * (s - c));
    if (S > 0)
        cout << "Obsah trojuhelnika je " << S << endl;
    else
        cout << "Nejedna se o platny trojuhelnik." << endl;
        }
        else
        cout << "Nejedna se o platny trojuhelnik."


    return 0;
}