#include<iostream>
#include<string>
#include<cmath>
#include<set>
using namespace std;

int main(){
    long long N,K;
    cin >> N >> K;
    if(K==0||N<K)   cout << 0 << endl;
    else if(N/2+1==K)   cout << N-1 << endl;
    else    cout <<  N-2 << endl;
    return 0;
}