#include<iostream>
#include<cstdio>
#include<cstdlib>
using namespace std;

int main() {
  long long int n,k;
  long long int cnt=0;
  cin>>n>>k;
  long long int A[n];
  for(int i=0;i<n;i++) {
    A[i] = 0;
    A[k] = 1; A[n-k+1] = 1;
    if(A[i] == 0) cnt = cnt + 1;
  }
  if((n==1 && k==1) || (n==2 && k==2)) cout<<0<<endl;
  else if(n-k < 0 || k == 0) cout<<0<<endl;
  else cout<<cnt<<endl;

  return 0;
}