B

#include <bits/stdc++.h>

using namespace std;

#define ll     long long
#define _test   int _TEST; cin>>_TEST; while(_TEST--)
#define ff     first
#define pb     push_back

int main()
{
    int n;
    cin>>n;

    vector<int> a(n);
    for(auto &e: a)
      cin>>e;

    sort(a.begin(), a.end());

    for(int i=1; ;i++)
    {
        if(a[i]-1 != a[i-1])
        {
            cout<<a[i]-1<<"\n";
            break;
        }
    }
}