#define _USE_MATH_DEFINES
#include<stdio.h>
#include<string>
#include<iostream>
#include<cctype>
#include<cstdio>
#include<vector>
#include<stack>
#include<queue>
#include <algorithm>
#include<math.h>
#include<set>
#include<map>
#include<iomanip>

//#include<bits/stdc++.h>


using namespace std;


int main() {
	
	long long int n, x,y;
	cin >> n >> x;
	long long int ans=n*x;
	if(n!=x)ans /= 2;
	else ans /= 4;
	if (n*x % 2)ans++;
	
	cout << ans-1 << endl;

	return 0;

}