#include<iostream>
#include<algorithm>
#include<cmath>
using namespace std;
int main() {
  int n,k;
  cin >> n >> k;
  if(k==0||k>n) {
      cout << 0 << endl;
  }
  else if(n%2==1&&(n+1)/2==k) {
      cout << n-1 << endl;
  }
  else {
      cout << n-2 << endl;
  }
}