#include <bits/stdc++.h>
using namespace std;
long long a[100100], n, d = 1e9, r, b[5], i, c = 1;
int main() {
  cin >> n;
  for (i = 0; i < n; ++i) cin >> a[i];
  sort(a, a + n);
  if (n == 1)
    cout << -1;
  else {
    for (i = 1; i < n; ++i)
      if (a[i - 1] != a[i]) c = 0;
    if (c)
      b[r++] = a[0];
    else {
      if (n == 2 && (d = a[1] - a[0]) % 2 == 0) b[r++] = a[0] + d / 2;
      for (i = 1; i < n; ++i) d = min(d, a[i] - a[i - 1]);
      for (i = 1; i < n; ++i)
        if (a[i] - a[i - 1] != d) {
          if (a[i] - a[i - 1] != 2 * d) {
            r = 1e9;
            break;
          }
          b[r++] = a[i - 1] + d;
        }
      if (r > 1)
        r = 0;
      else if (!r || (r == 1 && n == 2)) {
        b[r++] = a[0] - d;
        b[r++] = a[n - 1] + d;
      }
    }
    sort(b, b + r);
    cout << r << endl;
    for (i = 0; i < r; ++i) cout << b[i] << " ";
  }
}
