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