#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
using lli = long long int;
using vi = vector<int>;
using vll = vector<long long int>;
int main() {
  int N;
  cin >> N;
  int arr[N + 1];
  for (int i = 0; i < N; i++) {
    cin >> arr[i];
  }
  sort(arr, arr + N);
  if (N == 1) {
    cout << "-1"
         << "\n";
    return 0;
  }
  if (N == 2) {
    if (arr[0] == arr[1]) {
      cout << 1 << "\n";
      cout << arr[0] << "\n";
    } else {
      int cd = arr[1] - arr[0];
      cout << 2 + (cd % 2 == 0) << "\n";
      if (cd % 2 == 0)
        cout << arr[0] - cd << " " << arr[0] + cd / 2 << " " << arr[1] + cd
             << "\n";
      else
        cout << arr[0] - cd << " " << arr[1] + cd << "\n";
    }
    return 0;
  }
  set<int> s;
  for (int i = 1; i < N; i++) {
    s.insert(arr[i] - arr[i - 1]);
  }
  if (s.size() > 2) {
    cout << "0"
         << "\n";
    return 0;
  }
  if (s.size() == 1) {
    if (arr[0] == arr[N - 1]) {
      cout << 1 << "\n";
      cout << arr[0] << "\n";
      return 0;
    }
    cout << 2 << "\n";
    cout << arr[0] - *s.begin() << " " << arr[N - 1] + *s.begin() << "\n";
    return 0;
  }
  int diff = *s.rbegin();
  int cd = *s.begin();
  int cnt = 0, ans = 0;
  for (int i = 1; i < N; i++) {
    if (arr[i] - arr[i - 1] == diff && diff / 2 == cd) {
      cnt++;
      ans = arr[i - 1] + cd;
    }
  }
  if (diff % 2 != 0 || cnt != 1) {
    cout << "0"
         << "\n";
  } else {
    cout << 1 << "\n";
    cout << ans << "\n";
  }
  return 0;
}
