#include<iostream>
#include<vector>
#include<algorithm>

using namespace std;
using ll = long long;


int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    ll n,k;
    cin>>n>>k;
    if(k==0||k>n) {
        cout<<0<<endl;
        return 0;
    }
    ll ans = 0;
    if(k==n-k+1){
        ans = n - 1;
    }else{
        ans = n - 2;
    }
    ans = max(ans,0ll);
    cout<<ans<<endl;
}