#include <bits/stdc++.h>
using namespace std;
long long int a[100001];
long long int count1[100001];
int main() {
  long long int n, i, j, max = -100000000, min = 100000000, pos = 0, d;
  cin >> n;
  for (i = 0; i < n; i++) {
    cin >> a[i];
  }
  sort(a, a + n);
  if (n == 1)
    cout << "-1" << endl;
  else if (n == 2) {
    if (a[0] != a[1]) {
      d = a[1] - a[0];
      if (d % 2 == 0) {
        cout << "3\n";
        cout << a[0] - d << " ";
        cout << a[0] + d / 2 << " ";
        cout << a[1] + d << " ";
      } else {
        cout << "2\n";
        cout << a[0] - d << " ";
        cout << a[1] + d << " ";
      }
    } else {
      cout << "1\n";
      cout << a[0] << endl;
    }
  } else {
    for (j = 0; j < n - 1; j++) {
      d = a[j + 1] - a[j];
      count1[d] = count1[d] + 1;
      if (d > max) {
        max = d;
        pos = j;
      }
      if (d < min) {
        min = d;
      }
    }
    if (max == min) {
      if (max == 0) {
        cout << "1\n";
        cout << a[0] << endl;
      } else {
        cout << "2\n";
        cout << a[0] - min << " ";
        cout << a[n - 1] + min << " ";
      }
    } else if (max != min) {
      if (min == 0) {
        cout << "0";
      } else if ((max / min) != 2) {
        printf("0");
      } else if ((max / min) == 2) {
        if (count1[max] > 1) {
          printf("0");
        } else {
          printf("1\n");
          cout << a[pos] + min << " ";
        }
      }
    }
  }
}
