#include <bits/stdc++.h>
using namespace std;

int main(){
	long long int l,k;
	cin >> l >> k;
	if(l == k * 2) cout << 0 << endl;
	else if((l/2) % k == 0) cout << l / 2 - k << endl;
 	else cout << (l / 2) / k * k << endl;
}