#include <bits/stdc++.h>
#pragma GCC target("avx2")
#pragma GCC optimization("O3")
#pragma GCC optimization("unroll-loops")
using namespace std;
const int maxn = 2e5 + 5;
const double PI = 3.14159265358979323846;
void solve() {
  long long n;
  cin >> n;
  vector<long long> v(n);
  set<long long> s1;
  for (long long i = 0; i < n; i++) {
    cin >> v[i];
    if (s1.find(v[i]) != s1.end() && s1.size() > 1) {
      cout << 0 << '\n';
      return;
    }
    s1.insert(v[i]);
  }
  if (n == 1) {
    cout << -1 << '\n';
    return;
  }
  if (s1.size() == 1) {
    cout << 1 << '\n';
    cout << v[0] << '\n';
    return;
  }
  sort(v.begin(), v.end());
  set<long long> s2;
  unordered_map<long long, long long> amp;
  long long d = 0;
  long long d2 = INT_MAX;
  for (long long i = 1; i < n; i++) {
    s2.insert(v[i] - v[i - 1]);
    amp[v[i] - v[i - 1]]++;
    ;
    d = ((d) > (v[i] - v[i - 1]) ? (d) : (v[i] - v[i - 1]));
    d2 = ((d2) < (v[i] - v[i - 1]) ? (d2) : (v[i] - v[i - 1]));
  }
  if (s2.size() > 2) {
    cout << 0;
    return;
  }
  if (s2.size() == 1) {
    if (n == 2 && d / 2 == double(d) / 2) {
      vector<long long> ans;
      ans.push_back(v[0] - d);
      ans.push_back(v[n - 1] + d);
      ans.push_back(v[0] + d / 2);
      sort(ans.begin(), ans.end());
      cout << ans.size() << '\n';
      cout << ans[0] << " " << ans[1] << " " << ans[2];
      return;
    }
    cout << 2 << '\n';
    cout << v[0] - d << " " << v[n - 1] + d;
    return;
  }
  if (amp[d] > 1) {
    cout << 0 << '\n';
    return;
  }
  for (long long i = 1; i < n; i++) {
    if (v[i] - v[i - 1] == d && (double)d / 2 == d2) {
      cout << 1 << '\n';
      cout << v[i - 1] + d2 << '\n';
      return;
    }
  }
  cout << 0 << '\n';
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long l = 1;
  while (l--) {
    solve();
  }
  return 0;
}
