#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  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);
  unordered_map<long long, long long> ma;
  for (long long i = 0; i < n - 1; i++) {
    long long temp = arr[i + 1] - arr[i];
    ma[temp]++;
  }
  set<long long> ans;
  if (ma.size() == 1) {
    ans.insert(arr[0] - (ma.begin()->first));
    ans.insert(arr[n - 1] + (ma.begin()->first));
  }
  for (long long i = 0; i < n - 1; i++) {
    long long temp = arr[i + 1] - arr[i];
    if (temp % 2 != 0) continue;
    if (temp == 0) continue;
    if (ma[temp] == 1 && ma[temp / 2] == n - 2) {
      ans.insert((arr[i] + arr[i + 1]) / 2);
      break;
    }
  }
  cout << ans.size() << "\n";
  for (auto it = ans.begin(); it != ans.end(); it++) {
    cout << (*it) << " ";
  }
  return 0;
}
