#include <stdio.h>

int main(void) {
	int L, K;
	int Y;
	Y = 0;

	scanf("%d %d", &L, &K);
	
	Y = K * (L / (2 * K));
	if((L % (2*K)) == 0) {
		Y -= K;
	}
	
	printf("%d\n",Y);
	
	return 0;
}