#include <iostream>
#include <vector>
#include <cstdio>
#include <sstream>
#include <map>
#include <string>
#include <algorithm>
#include <queue>
#include <cmath>

using namespace std;


int main(){
	long long r,c;
	cin >> r >> c;
	long long ans;
	if(r==c){
		if(r%2 == 1){
			ans = (r*c-1)/4 + 1;
		}else{
			ans = (r*c)/4;
		}
	}else{
		if(r%2 == 1 && c%2 == 1){
			ans = (r*c-1)/2 + 1;
		}else{
			ans = r*c/2;
		}
	}
	cout << ans-1 << endl;
	return 0;
}