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