#include<bits/stdc++.h>
using namespace std;

int main(){
    int n,k;
    cin >> n >> k;
    if(k<1||k>n) cout << 0 << endl;
    else if(n/2+1==k&&n%2!=0) cout << n-1 << endl;
    else cout << n-2 << endl;

    return 0;
}