#include <iostream>
#include <algorithm>
#include <regex>
using namespace std;

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

    return 0;
}