#include<bits/stdc++.h>

using namespace std;

typedef long long int64;

int main()
{
  int N, K;
  cin >> N >> K;
  if(K == 0 || N < K) {
    cout << 0 << endl;
  } else if(((N + 1) / 2 == K && N % 2 == 1)) {
    cout << N - 1 << endl;
  } else {
    cout << N - 2 << endl;
  }
}