#include <stdio.h>

int main() {
	int r,c;
	scanf("%d%d",&r,&c);
	if (r==c) {
		printf("%lld\n",r%2?(long long)r*r/4:(long long)r*r/4-1);
	} else {
		if (r%2==0||c%2==0) {
			printf("%lld\n",(long long)r*c/2-1);
		} else {
			printf("%lld\n",(long long)r*c/2);
		}
	}
    return 0;
}