#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  while (scanf("%d%d", &n, &m) != EOF) {
    if (n == m)
      cout << 0 << endl;
    else if (m == 0)
      cout << 1 << endl;
    else {
      if (m <= n / 2)
        cout << m << endl;
      else
        cout << n - m << endl;
    }
  }
}
