#include <bits/stdc++.h>
using namespace std;
priority_queue<int, vector<int>, less<int> > num;
int main() {
  int n, x;
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> x;
    num.push(x);
  }
  cout << ((num.top()) xor (x)) << "\n";
}
