#include <iostream>
using namespace std;

int main() {
  int n, k;
  cin >> n >> k;

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