#include<iostream>
#include<string>
#include<math.h>
using namespace std;

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