#include <iostream>

using namespace std;

int main(){
  int L, K, cnt=0;
  cin >> L >> K;
  for (int i=0, j=L; i<L/2; i+=K, j-=K) {
    if (i<j) cnt++;
  }
  cout << ((cnt-1)*K>=0 ? (cnt-1)*K: 0) << endl;
}