#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 cout << (n - k == k + 1 ? n - 2 : n - 1) << endl;  
}