#include<cstdio>
#include<algorithm>

typedef long long ll;

int main()
{
	ll R, C;
	scanf( "%lld%lld", &R, &C );

	printf( "%lld\n", std::max( 0ll, (R*C>>(R==C?2:1))-!(R*C&1) ) );

	return 0;
}