#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 50;
const long long M = 1e9 + 7;
long long gcd(long long a, long long b) { return !b ? a : gcd(b, a % b); }
long long lcm(long long a, long long b) { return (a / gcd(a, b)) * b; }
inline int add(int a, int b) { return a + b >= M ? a + b - M : a + b; }
inline int mul(int a, int b) { return (long long)a * b % M; }
map<int, int> f;
void solve() {
  int n;
  scanf("%d", &n);
  vector<int> v(n);
  for (int i = 0; i < n; ++i) scanf("%d", &v[i]);
  sort(v.begin(), v.end());
  if (n == 1) {
    puts("-1");
    return;
  } else if (v[0] == v[n - 1]) {
    printf("1\n%d", v[0]);
    return;
  } else if (n == 2) {
    int d = (v[1] - v[0]);
    if (d % 2 == 0)
      printf("3\n%d %d %d", v[0] - d, v[0] + d / 2, v[1] + d);
    else
      printf("2\n%d %d", v[0] - d, v[1] + d);
    return;
  }
  set<int> d;
  for (int i = 1; i < n; ++i) {
    d.insert(v[i] - v[i - 1]);
    ++f[v[i] - v[i - 1]];
  }
  if (d.size() == 1) {
    printf("2\n%d %d", v[0] - *d.begin(), v[n - 1] + *d.begin());
  } else if (d.size() > 2) {
    puts("0");
  } else {
    auto i = d.begin();
    int d1 = *i;
    ++i;
    int dd = *i;
    if (d1 * 2 != dd || dd % 2 || f[dd] > 1) {
      puts("0");
      return;
    }
    for (int i = 1; i < n; ++i) {
      if ((v[i] - v[i - 1]) == dd) {
        printf("1\n%d", v[i - 1] + dd / 2);
        return;
      }
    }
  }
}
int main() {
  int t = 1;
  while (t--) solve();
}
