#include <bits/stdc++.h>
using namespace std;
long long MOD = 998244353;
double eps = 1e-12;
void solve() {
  int n;
  cin >> n;
  vector<int> v;
  int temp;
  for (int i = 0; i < n; i++) {
    cin >> temp;
    v.push_back(temp);
  };
  sort((v).begin(), (v).end());
  if (n == 1) {
    cout << -1 << "\n";
  } else if (n == 2) {
    int d = v[1] - v[0];
    if (d % 2) {
      cout << 2 << "\n";
      cout << v[0] - d << " " << v[1] + d;
    } else {
      if (d == 0) {
        cout << 1 << "\n";
        cout << v[0];
      } else {
        cout << 3 << "\n";
        cout << v[0] - d << " " << v[0] + d / 2 << " " << v[1] + d;
      }
    }
  } else {
    map<int, int> m;
    set<int> s;
    for (int i = 0; i < n - 1; i++) {
      s.insert(v[i + 1] - v[i]);
    }
    if (s.size() == 1) {
      if (v[1] - v[0] == 0) {
        cout << 1 << "\n";
        cout << v[0];
      } else {
        cout << 2 << "\n";
        cout << v[0] - (v[1] - v[0]) << " " << v[n - 1] + (v[1] - v[0]);
      }
    } else if (s.size() == 2) {
      auto it = s.begin();
      int x = *it;
      it++;
      int y = *it;
      if (x * 2 == y) {
        int count = 0;
        int k;
        for (int i = 0; i < n - 1; i++) {
          if (v[i + 1] - v[i] == y) {
            k = i;
            count++;
          }
        }
        if (count == 1) {
          cout << 1 << "\n";
          cout << v[k] + x;
        } else {
          cout << 0 << "\n";
        }
      } else {
        cout << 0 << "\n";
      }
    } else {
      cout << 0 << "\n";
    }
  }
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
