#include <stdio.h>

int main(void){
	int l, k, m, cnt;
	
	scanf("%d%d", &l, &k);
	l *= 10;
	k *= 10;
	
	l /= 2;
	
	m = l / k;
	
	if(m * k == l){
		cnt = (m - 1) * k;
	}else{
		cnt = m * k;
	}
	
	cnt /= 10;
	
	printf("%d", cnt);
	
	return 0;
}