#include <bits/stdc++.h>
using namespace std;
#pragma comment(linker, "/STACK:102400000,102400000")
long long mul(long long a, long long b) { return (a * b) % (1000000007); }
long long add(long long a, long long b) { return (a + b) % (1000000007); }
long long sub(long long a, long long b) {
  return ((a - b) % (1000000007) + (1000000007)) % (1000000007);
}
void upd(long long &a, long long b) {
  a = (a % (1000000007) + b % (1000000007)) % (1000000007);
}
int read() {
  int x = 0, f = 1;
  char ch = getchar();
  while (!isdigit(ch)) {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (isdigit(ch)) {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
int a[100000];
int main() {
  int n = read();
  for (int i = 1; i <= n; i++) cin >> a[i];
  int p = *max_element(a + 1, a + 1 + n);
  cout << (p ^ a[n]) << endl;
  return 0;
}
