#include <stdio.h>
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <functional>
#include <map>
#include <iomanip>
#include <math.h> 
#include <stack>
#include <queue>
#include <bitset>
#include <cstdlib>
#include <tuple>
#include <cctype>
#include <ctype.h>
#include <set>
#include <sstream>

using namespace std;


int main() {
	int i, j, k;
	long l;

	cin >> l >> k;

	long l2 = l / 2;

	long l3 = l2 / k;

	//cout << (l - (l3 * k)) << endl;

	if ((l3*k) * 2 == l) {
		l3--;
		if (l3 <= 0) {
			cout << 0 << endl;
		}
		else {
			cout << l3 * k << endl;
		}
	}
	else {

		cout << k * l3 << endl;

	}


	


	getchar();
	getchar();
	return 0;
}