#include <iostream>		//cin, cout
#include <vector>		//vector
#include <algorithm>	//sort,min,max,count
#include <string>		//string,getline, to_string
#include <cstdlib>		//abs(int)
#include <utility>		//swap, pair

using namespace std;

int main() {

	int L, K;
	cin >> L >> K;

	int ans;
	int mod;

	if (L <= K * 2) {
		ans = 0;
	}
	else {
		ans = (L / (2 * K)) * K;
		mod = L % (2 * K);
		if (mod == 0) {
			ans -= K;
		}
	}

	cout << ans << endl;

	return 0;

}