#include <iostream>

using namespace std;

int main() {

  int l,k;
  cin >> l >> k;
  for (int i = 0; i < 2147483646 ; i++) {
    if(l - 2 * k * i <= 0){
      cout << k * (i-1) <<"\n";
      break;
    }else if (l - 2 * k == 0) {
      cout << 0 << "\n";
      break;
    }
  }


  return 0;
}