#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint1000000007;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 100000000000000000


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