#include <iostream>
using namespace std;
int main(int argc, char* argv[])
{
	int N,K;
	cin>>N>>K;
	if (K==0 || K>N){
		cout<<0<<endl;
		return 0;
	}
	if (K==N){
		if (N<=2){
			cout<<0<<endl;
		}else{
			cout<<N-2<<endl;
		}
		return 0;
	}
	if (2*K==N+1){
		cout<<N-1<<endl;
	}else{
		cout<<N-2<<endl;
	}
	return 0;
}