#include<bits/stdc++.h>
using namespace std;
int main(){
    double n,m;
    cin>>n>>m;
    double i=(n*m)/2;
    printf("%.1lf",i);
    return 0;
}

C++ :

#include<bits/stdc++.h>
using namespace std;
double a,b;
int main(){
	cin>>a>>b;
    printf("%.1f",a*b/2);
    return 0;
}