#include <stdio.h>
#include <algorithm>
#include <deque>
using namespace std;
using ll = long long;

//63
int main() {
	int L, K, step;
	scanf("%d %d", &L, &K);
	step = L / (K * 2);
	step = step - (L % (K * 2) == 0);
	printf("%d\n", step * K);
}