#include <iostream>
#include <vector>
#include <algorithm>
#include <set>
#include <map>
using namespace std;

int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(0);

  int n,k;
  cin >> n >> k;
  if(k > n || k == 0) cout << 0 << endl;
  else if(n % 2 == 1 && k == (n+1)/2) cout << n - 1 << endl;
  else cout << n - 2 << endl;
}