#include <bits/stdc++.h>
using namespace std;
const int mod = 1e9 + 7;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int n;
  cin >> n;
  long long int arr[n];
  set<long long int> diff, all;
  for (long long int i = 0; i < n; i++) {
    cin >> arr[i];
    all.insert(arr[i]);
  }
  sort(arr, arr + n);
  if (n == 1) {
    cout << -1;
    return 0;
  } else if (n == 2) {
    if (arr[1] == arr[0])
      cout << "1\n" << arr[0] << "\n";
    else if ((arr[1] - arr[0]) % 2 == 0)
      cout << "3\n"
           << arr[0] - (arr[1] - arr[0]) << " " << (arr[1] + arr[0]) / 2 << " "
           << arr[1] + (arr[1] - arr[0]);
    else
      cout << "2\n"
           << arr[0] - (arr[1] - arr[0]) << " " << arr[1] + (arr[1] - arr[0]);
    return 0;
  }
  for (long long int i = 1; i < n; i++) diff.insert(arr[i] - arr[i - 1]);
  if (diff.size() == 1) {
    if (all.size() == 1)
      cout << "1\n" << arr[0];
    else
      cout << "2\n"
           << arr[0] - (arr[1] - arr[0]) << " "
           << arr[n - 1] + (arr[1] - arr[0]);
  } else if (diff.size() > 2) {
    cout << 0;
    return 0;
  } else {
    int counter = 0, index;
    long long int first = *(diff.begin());
    long long int second = *(++diff.begin());
    if (first * 2 == second) {
      for (long long int i = 1; i < n; i++) {
        if (arr[i] - arr[i - 1] == second) {
          ++counter;
          index = i;
        }
      }
      if (counter == 1) {
        cout << "1\n" << (arr[index] + arr[index - 1]) / 2;
        return 0;
      } else {
        cout << 0;
        return 0;
      }
    } else {
      cout << 0;
      return 0;
    }
  }
  return 0;
}
