#include<cstdio>

using ll=long long;

int main(void)
{
	int r, c;

	while(scanf("%d%d", &r, &c)==2)
	{
		ll ans;
		if(r&1 && c&1) ans=(ll)r*(c-1)/2+(r+1)/2;
		else ans=(ll)r*c/2;

		if(r==c)
		{
			ans=(ll)r*c/4+(r&1);
		}

		printf("%lld\n", ans-1);
	}

	return 0;
}