#include <iostream>
#include <sstream>
#include <math.h>
#include <stack>
#include <set>
#include <algorithm>
#include <string>
#include <queue>
#include <stack>
#include <map>
#include <cstdio>
#include <vector>
using namespace std;

#define INF (1 << 30)
#define INFLL (1LL << 60)

int main() {
	int a,b;
	cin >> a >> b;
	for(int i = 0;;i++){
		if(a <= 2*b*i) {
			cout << b * (i-1) << endl;
			break;
		}
	}
	return 0;
}