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

int main() {
	int L, K;
	cin >> L >> K;

	int b = L / K;
	b /= 2;
	int a = 0;
	if (L % K != 0) {
		a = K * b;
	}
	else {
		if ((b * 2 * K) == L) {
			b--;
			a = K * b;
		}
		else {
			a = K * b;
		}
	}

	cout << a << endl;
}