#define ll long long

#include<iostream>

using namespace std;

int main(){
	ll n,k;
	cin>>n>>k;
	if(k==0 | k>n){
		cout<<0<<endl;
	}else if(k*2-1==n){
		cout<<n-1<<endl;
	}else{
		cout<<n-2<<endl;
	}
	return 0;
}