#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long n;
  cin >> n;
  long long a[n];
  for (long long i = 0; i < n; ++i) {
    cin >> a[i];
  }
  sort(a, a + n);
  long long d;
  long long c = 0, x, b;
  if (n == 1)
    cout << "-1";
  else if (n == 2) {
    if (a[1] == a[0]) {
      cout << "1" << endl;
      cout << a[0];
    } else if ((a[1] - a[0]) % 2 == 0) {
      cout << "3" << endl;
      d = a[1] - a[0];
      cout << a[0] - d << " " << (a[1] + a[0]) / 2 << " " << a[1] + d;
    } else {
      cout << "2" << endl;
      d = a[1] - a[0];
      cout << a[0] - d << " " << a[1] + d;
    }
  } else {
    d = a[1] - a[0];
    if (d > a[2] - a[1]) d = a[2] - a[1];
    for (long long i = 0; i < n - 1; ++i) {
      if (a[i + 1] - a[i] != d) {
        c++;
        x = i;
      }
    }
    if (c == 0) {
      if (d == 0) {
        cout << "1" << endl;
        cout << a[0];
        return 0;
      }
      cout << "2" << endl;
      cout << a[0] - d << " " << a[n - 1] + d;
    } else if (c == 1) {
      b = a[x + 1] - a[x];
      if (b % 2 == 0 && d == b / 2) {
        cout << "1" << endl;
        cout << a[x] + b / 2;
      } else
        cout << "0";
    } else
      cout << "0";
  }
  return 0;
}
