// yukicoder: No.84 悪の算盤
// 2019.6.22 bal4u

#include <stdio.h>

int main()
{
	int R, C;
	long long ans;

	scanf("%d%d", &R, &C);
	ans = (long long)R * C;
	if (R == C) printf("%lld\n", (ans >> 2) - 1 + (ans & 1));
	else printf("%lld\n", (ans >> 1) - 1 + (ans & 1));
	return 0;
}