#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a[n + 1];
  for (int i = 1; i <= n; ++i) cin >> a[i];
  int x = a[2] - a[1];
  for (int i = 2; i < n; ++i)
    if (a[i + 1] - a[i] != x) {
      x = -1;
      break;
    }
  cout << (x != -1 ? a[n] + x : a[n]);
  return 0;
}
