#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int n, i, q;
  cin >> n;
  long long int arr[100005];
  for (i = 0; i < n; i++) {
    cin >> arr[i];
  }
  sort(arr, arr + n);
  if (n == 1) {
    cout << "-1";
    return 0;
  }
  if (arr[0] == arr[n - 1]) {
    cout << "1"
         << "\n";
    cout << arr[0];
    return 0;
  }
  if (n == 2) {
    int diff = arr[1] - arr[0];
    if ((arr[0] + arr[1]) % 2 == 0) {
      cout << "3"
           << "\n";
      cout << arr[0] - (diff) << " " << (arr[0] + arr[1]) / 2 << " "
           << arr[1] + (diff);
      return 0;
    } else {
      cout << "2"
           << "\n";
      cout << arr[0] - diff << " " << arr[1] + diff;
      return 0;
    }
  } else {
    int temp = 0;
    q = 0;
    int diff = min((arr[1] - arr[0]), (arr[2] - arr[1]));
    for (i = 1; i < n; i++) {
      if ((arr[i] - arr[i - 1]) != diff) {
        q++;
        temp = i;
      };
    }
    if (q > 1) {
      cout << "0";
      return 0;
    } else {
      if (q == 1) {
        if ((arr[temp] - arr[temp - 1]) == 2 * diff) {
          cout << "1"
               << "\n";
          cout << arr[temp] - diff;
          return 0;
        } else {
          cout << "0";
          return 0;
        }
      }
      if (q == 0) {
        cout << "2"
             << "\n";
        cout << arr[0] - diff << " " << arr[n - 1] + diff;
        return 0;
      }
    }
  }
}
