#include <bits/stdc++.h>

using namespace std;

#define REP(i,a,b) for(int i=a;i<(int)b;i++)
#define rep(i,n) REP(i,0,n)

int main() {

  int L, K; cin >>L>>K;
  int a = L-L/(K<<1)*K;
  if(a<=0) cout << 0 << endl;
  else cout << a << endl;
  return 0;
}