#define REP(i, n) for(int i = 0;i < n;i++)
#define REPR(i, n) for(int i = n;i >= 0;i--)
#define FOR(i, m, n) for(int i = m;i < n;i++)

#include <iostream>
#include <string>
#include <algorithm>
using namespace std;
int main() {
	int ans,a,b;
	cin >> a >> b;

	ans = b*(a/(2*b));
	if(a%(2*b) == 0) ans -= b;
	cout << ans << endl;
	return 0;
}