#include <stdio.h>

int main(void) {
	int l, L, K, eat;
	scanf("%d%d", &L, &K);
	l = L;
	eat = 0;
	while(l > 0){
		if(l - 2 * K <= 0){
			break;
		}
		l -= 2 * K;
		eat += K;
	}
	printf("%d\n", eat);
	
	return 0;
}