#include <bits/stdc++.h>
using namespace std;
int main(){
	int r,c;	cin>>r>>c;
	if(r%2==1&&c%2==1){
		cout<<(long long int)(r/2)*c+c/2<<endl;
	}
	else cout<<(long long int)(r*c/2-1)<<endl;
	return 0;
}