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

int main(){
  int n, a, ans = 0;
  cin >> n;
  map<int, int> mp;
  for (int i = 0; i < n; i++){
    cin >> a;
    mp[a]++;
  }
  for (auto [f, s] : mp){
    if (s == 1) ans = f;
  }
  cout << ans << endl;
  return 0;
}