#include<iostream>

using namespace std;

int main(){

	int N,K;
	int NO=0;

	cin>>N>>K;

	if(N+1-K<=N&&N+1-K>=1) NO++;
	if(K>=1&&K<=N) NO++;

	if(NO==0) cout<<"0"<<endl;
	else if(NO==1) cout<<N-1<<endl;
	else {
		if(N+1-K==K) cout<<N-1<<endl;
		else cout<<N-2<<endl;
	}

}