#include <iostream>
using namespace std;

int main() {
	int L, K; cin >> L >> K;
	int cut, rest;
	cut = L / (2*K);
	rest = L % (2*K);
	if (rest == 0){
		cout << (cut -1)*K << endl;
	} else {
		cout << cut * K << endl;
	}
	return 0;
}