#include<iostream>
#include<string>
#include<fstream>
#include<iomanip>
#include<vector>
#include<cctype>
#include<algorithm>
#include<sstream>
using namespace std;

int main()
{
	int a, b;
	cin >> a >> b;
	if (a % (2 * b))cout << b*(a / (2 * b)) << endl;
	else cout << b*(a / (2 * b) - 1) << endl;
	return 0;
}