#include<bits/stdc++.h>
using namespace std;
int main(){
  int n,k; cin>>n>>k;
  int ans;
  if(k>n || k==0) ans=0;
  else{
    if(n%2==1&&k==n/2+1) ans=n-1;
    else ans=n-2;
  }
  cout<<ans<<endl;
  return 0;
}