#include<bits/stdc++.h>
#define int long long
using namespace std;


signed main(){
	int a, b;
	cin >> a >> b;
	int ans;
	if(a == b)ans = (a * (a + 1)) / 2;
	else ans = (a * b + 1) / 2;

	cout << ans;
	return 0;
}

想知道哪里有问题，求大佬解答