#include<iostream>
using namespace std;
int main(){
   int n,k,ans;
   cin >> n>>k;
   if(k==0){ans=0;
   }else if(k>n){ans=0;
    }else if(k*2==n+1){ans=n-1;
    }else{ ans=n-2;
}
cout<<ans<<endl;
return 0;
}