#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  int tc = 1;
  while (tc--) {
    cin >> n >> m;
    if (m == 0)
      cout << 1 << endl;
    else {
      if (m <= n / 2)
        cout << m << endl;
      else
        cout << (n - m) << endl;
    }
  }
}
