#include <bits/stdc++.h>
using namespace std;
long long mod = 1e9 + 7;
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  long long n;
  cin >> n;
  long long arr[n];
  for (long long i = 0; i < n; i++) cin >> arr[i];
  if (n == 1) {
    cout << -1;
    return 0;
  }
  sort(arr, arr + n);
  long long d = arr[1] - arr[0];
  if (n == 2 && (d % 2) == 0 && d != 0) {
    cout << 3 << "\n";
    cout << arr[0] - d << " " << arr[0] + d / 2 << " " << arr[n - 1] + d;
    return 0;
  }
  long long ans;
  long long re = 0;
  if (n == 3 && (arr[2] - arr[1]) != d) {
    if ((arr[1] - (arr[0] + arr[2] - arr[1])) == (arr[2] - arr[1])) {
      cout << 1 << "\n";
      cout << arr[0] + arr[2] - arr[1];
      return 0;
    }
  }
  if (n > 3 && ((arr[n - 1] - arr[n - 2]) != d &&
                (arr[n - 2] - arr[n - 3]) == (arr[n - 1] - arr[n - 2])))
    d = (arr[n - 1] - arr[n - 2]);
  for (long long i = 0; i < n - 1; i++) {
    if ((arr[i + 1] - arr[i]) != d &&
        (((arr[i + 1] - arr[i]) != (2 * d)) ||
         (arr[i + 1] - arr[i]) == (2 * d) && re == 1)) {
      cout << 0;
      return 0;
    } else if ((arr[i + 1] - arr[i]) == (2 * d)) {
      re = 1;
      ans = arr[i] + d;
    }
  }
  if (re == 1) {
    cout << 1 << "\n";
    cout << ans;
    return 0;
  }
  cout << 2 << "\n";
  cout << arr[0] - d << " " << arr[n - 1] + d;
}
