#include <bits/stdc++.h>
using namespace std;
int t, i, n, y, a[102400];
map<int, int> C, X, R;
int main() {
  for (cin >> n; i < n; cin >> a[i++])
    ;
  if (n == 1) {
    cout << -1;
    return 0;
  }
  for (sort(a, a + n); --i;) y = a[i] - a[i - 1], C[y]++, X[y] = i;
  y = C.begin()->first;
  t = C.rbegin()->first;
  if (y == t)
    R[a[0] - y] = R[a[n - 1] + y] = n > 2 || y % 2 ? 1 : R[a[1] - y / 2] = 1;
  if (C.size() == 2 && t == y * 2 && C[t] == 1) R[a[X[t]] - y] = 1;
  cout << R.size() << endl;
  for (auto r : R) cout << r.first << " ";
}
