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