#include <iostream>
#include <vector>

using namespace std;

int main(){
	long l;
	int k, a;
	cin >> l,cin >> k;
	if (l / k == 1 || k * 2 == l){
		a = 0;
	}
	else{
		a = (l / (k * 2)) * k;
		if (a * 2 == l){
			a -= k;
		}
	}
	cout << a << endl;
	return 0;
}