c#include <bits/stdc++.h> using namespace std; int main(){ cin.tie(NULL); cout.tie(NULL); ios_base::sync_with_stdio(false); int N; cin >> N; map<int, int> cnt; for(int i = 0, n; i < N; i++){ int input; cin >> input; cnt[input]++; } for(int i = 50; i >= 0; i--){ if(cnt[i] == i){ cout << i << "\n"; return 0; } } cout << -1 << "\n"; return 0; }