#include <bits/stdc++.h>
using namespace std;
int main(void) {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int n;
  cin >> n;
  int a[n];
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  sort(a, a + n);
  if (n == 1) {
    cout << "-1\n";
    return 0;
  }
  if (n == 2) {
    int d = a[1] - a[0];
    if (d == 0) {
      cout << "1\n";
      cout << a[0] << endl;
      return 0;
    }
    vector<int> tmp;
    tmp.push_back(a[1] + d);
    tmp.push_back(a[0] - d);
    if (d % 2 == 0) {
      tmp.push_back((a[0] + a[1]) / 2);
    }
    sort(tmp.begin(), tmp.end());
    cout << tmp.size() << endl;
    for (int i = 0; i < tmp.size(); i++) {
      cout << tmp[i] << " ";
    }
    return 0;
  }
  int d1 = a[1] - a[0];
  int d2 = a[2] - a[1];
  int d = min(d1, d2);
  int indx = -1;
  for (int i = 0; i < n - 1; i++) {
    if (a[i + 1] - a[i] != d) {
      if (indx != -1 || a[i + 1] - a[i] != 2 * d) {
        cout << "0\n";
        return 0;
      }
      indx = i;
    }
  }
  if (d == 0) {
    cout << "1\n";
    cout << a[0] << endl;
    return 0;
  }
  if (indx != -1) {
    cout << "1\n";
    cout << a[indx] + d << endl;
    return 0;
  }
  cout << "2\n";
  cout << a[0] - d << " " << a[n - 1] + d << endl;
  return 0;
}
