#include<stdio.h>
int main(void){
	int l, k, cnt, mod, length;
	scanf("%d%d", &l, &k);
	cnt = l / k;
	mod = l % k;
	if(mod == 0 && cnt%2 == 0){
		printf("%d", k * (cnt/2-1));
	}else{
		printf("%d", k * (cnt/2));
	}
	return 0;
}