#include <bits/stdc++.h>
using namespace std;
vector<int> dif;
map<int, int> g;
int a[120000];
int main() {
  int n, flag;
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  sort(a + 1, a + 1 + n);
  for (int i = 2; i <= n; i++) {
    if (g[a[i] - a[i - 1]] == 0) dif.push_back(a[i] - a[i - 1]);
    g[a[i] - a[i - 1]]++;
  }
  int sz = dif.size();
  if (sz > 2 || n == 1) {
    if (sz > 2)
      cout << 0 << endl;
    else
      cout << -1 << endl;
    return 0;
  }
  if (sz == 1) {
    int D = dif[0];
    if (dif[0] == 0) {
      cout << 1 << endl;
      cout << a[1] << endl;
      return 0;
    }
    if (n == 2) {
      if ((a[2] - a[1]) % 2 == 0) {
        cout << 3 << endl;
        cout << a[1] - D << " " << (a[2] + a[1]) / 2 << " " << a[2] + D << endl;
        return 0;
      }
    }
    cout << 2 << endl;
    cout << a[1] - D << " " << a[n] + D << endl;
    return 0;
  }
  int D1 = min(dif[0], dif[1]), D2 = max(dif[0], dif[1]);
  if (D1 * 2 != D2 || g[D2] > 1) {
    cout << 0 << endl;
    return 0;
  }
  cout << 1 << endl;
  for (int i = 2; i <= n; i++) {
    if (a[i] - a[i - 1] == D2) {
      cout << a[i] - D1 << endl;
      return 0;
    }
  }
  return 0;
}
