#include <bits/stdc++.h>
using namespace std;
const long long N = 2e5 + 5;
long long n, m, k;
long long a[N];
void solve() {
  long long i, j;
  cin >> n;
  for (i = 0; i < n; i++) cin >> a[i];
  sort(a, a + n);
  set<long long> ans;
  if (n == 1) {
    cout << -1 << '\n';
    return;
  }
  bool good = true;
  for (i = 1; i < n; i++)
    if (a[i] - a[i - 1] != a[1] - a[0]) good = false;
  if (good) {
    long long diff = a[1] - a[0];
    ans.insert(a[0] - diff);
    ans.insert(a[n - 1] + diff);
    if (n == 2 && (a[1] + a[0]) % 2 == 0) ans.insert((a[1] + a[0]) / 2);
  } else {
    map<long long, long long> d;
    for (i = 1; i < n; i++) d[a[i] - a[i - 1]]++;
    for (i = 1; i < n; i++) {
      if ((a[i] - a[i - 1]) % 2 != 0) continue;
      long long diff = (a[i] - a[i - 1]) / 2;
      if (!diff) continue;
      if (d[diff] == n - 2) {
        ans.insert(a[i] - diff);
      }
    }
  }
  cout << ans.size() << '\n';
  for (auto x : ans) cout << x << ' ';
  cout << '\n';
  return;
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  solve();
  return 0;
}
