#include <bits/stdc++.h>
using namespace std;
long long n, m, k, a[1000000], x = 100000000, y, z;
vector<long long> v;
set<long long> s;
bool b = 0, bb = 0, bbb = 0;
int main() {
  cin >> n;
  for (int i = 0; i < n; i++) cin >> a[i];
  sort(a, a + n);
  for (int i = 1; i < n; i++) x = min(x, a[i] - a[i - 1]);
  if (n == 1) {
    cout << -1;
    return 0;
  }
  if (n == 2) {
    y = a[1] - a[0];
    if (y % 2 == 0 && y != 0)
      cout << 3 << endl << a[0] - y << " " << y / 2 + a[0] << " " << a[1] + y;
    else if (y == 0)
      cout << 1 << endl << a[0];
    else
      cout << 2 << endl << a[0] - y << " " << a[1] + y;
    return 0;
  }
  for (int i = 0; i < n - 1; i++) {
    k = x + a[i];
    if (k == a[i + 1]) continue;
    if (k + x == a[i + 1] && !b) {
      z = k;
      b = 1;
    } else {
      cout << 0;
      return 0;
    }
  }
  if (!b) {
    if (x == 0)
      cout << 1 << endl << a[0];
    else {
      cout << 2 << endl << a[0] - x << " " << a[n - 1] + x;
    }
  } else
    cout << 1 << endl << z;
  return 0;
}
