#include<bits/stdc++.h>
using namespace std;
int main(){
    double a,b;
    cin >> a >> b;
    cout<<"s="<<a*b/2;
    return 0;
}

#include <bits/stdc++.h>

using namespace std;

int main() {

double a, b;

cin >> a >> b;

cout <<"s=" <<a *b / 2;

return 0;

}