#include <iostream>
#include <iomanip>
#include <string>
#include <limits.h>
#include <math.h>
#include <vector>
#include <algorithm>
#include <map>
#include <numeric>
#include <cmath>

using namespace std;

int main(){
  long long l,k;
  cin >> l >> k;
  if(l == 2 * (l/(2*k) * k)){
  	cout << (l/(2*k) - 1) * k << endl;
  }else{
	  cout << l/(2*k) * k << endl;
  }
  return 0;

}