#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O2")
const long long MOD = 1e9 + 7;
const double PI = acos(-1);
long long add(long long x, long long y) {
  long long res = x + y;
  return (res >= MOD ? res - MOD : res);
}
long long mul(long long x, long long y) {
  long long res = x * y;
  return (res >= MOD ? res % MOD : res);
}
long long sub(long long x, long long y) {
  long long res = x - y;
  return (res < 0 ? res + MOD : res);
}
long long power(long long a, long long b, long long m = MOD) {
  long long ans = 1;
  a = a % m;
  while (b > 0) {
    if (b & 1) ans = (1ll * a * ans) % m;
    b >>= 1;
    a = (1ll * a * a) % m;
  }
  return ans;
}
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
long long lcm(long long x, long long y) { return (x * y) / gcd(x, y); }
bool isprime(long long n) {
  if (n < 2) return 0;
  long long i = 2;
  while (i * i <= n) {
    if (n % i == 0) return 0;
    i++;
  }
  return 1;
}
bool isPowerOfTwo(long long x) { return x && (!(x & (x - 1))); }
double distform(long long x, long long y, long long z, long long w) {
  return sqrt(1. * pow(x - z, 2) + 1. * pow(y - w, 2));
}
long long dx[] = {-1, 1, 0, 0, -1, 1, 1, -1};
long long dy[] = {0, 0, 1, -1, 1, 1, -1, -1};
const long long MAXN = 2e5 + 10;
const long long inf = 1e18;
template <typename T, typename T1>
T amax(T &a, T1 b) {
  if (b > a) a = b;
  return a;
}
template <typename T, typename T1>
T amin(T &a, T1 b) {
  if (b < a) a = b;
  return a;
}
long long intlog(double base, double x) {
  return (long long)(log(x) / log(base));
}
bool isvalid(long long x, long long y, long long n, long long m) {
  return x >= 0 and x < n and y >= 0 and y < m;
}
template <typename T>
void read(T &x) {
  for (long long i = 0; i < ((long long)(x).size()); ++i) cin >> x[i];
}
void solve() {
  long long n;
  cin >> n;
  vector<long long> a(n);
  read(a);
  sort(a.begin(), a.end());
  unordered_map<long long, long long> diff;
  for (long long i = 1; i < n; ++i) {
    diff[a[i] - a[i - 1]]++;
  }
  if (((long long)(diff).size()) > 2) {
    cout << 0 << endl;
    return;
  }
  long long minn = inf;
  long long diffatmin;
  long long other = 1;
  for (auto &x : diff) {
    if (minn > x.second) {
      minn = x.second;
      diffatmin = x.first;
    }
    if (minn == x.second) {
      if (diffatmin < x.first) diffatmin = x.first;
    }
  }
  for (auto &x : diff) {
    if (x.first == diffatmin) continue;
    other = x.first;
  }
  if (((long long)(diff).size()) > 1 and minn > 1) {
    cout << 0 << endl;
    return;
  }
  {
    cerr << " ["
         << "(diffatmin % 2 != 0)"
         << ": " << (diffatmin % 2 != 0) << "]\n";
  }
  {
    cerr << " ["
         << "(diffatmin / 2 != other)"
         << ": " << (diffatmin / 2 != other) << "]\n";
  }
  {
    cerr << " ["
         << "(other != 0)"
         << ": " << (other != 0) << "]\n";
  }
  if (((long long)(diff).size()) > 1 and
      (diffatmin % 2 != 0 or diffatmin / 2 != other or other == 0)) {
    cout << 0 << endl;
    return;
  }
  if (((long long)(diff).size()) == 2) {
    cout << 1 << endl;
    for (long long i = 1; i < n; ++i) {
      if (a[i] - a[i - 1] == diffatmin) {
        cout << a[i - 1] + diffatmin / 2 << endl;
        return;
      }
    }
  }
  if (a.size() == 1) {
    cout << -1 << endl;
    return;
  }
  bool ok = false;
  bool x = ((a[1] - a[0]) % 2 == 0);
  ok = ((long long)(a).size()) == 2 and ((a[1] - a[0]) % 2 == 0);
  if (diff.begin()->first == 0) {
    cout << 1 << endl;
    cout << a[0] << endl;
    return;
  }
  cout << 2 + ok << endl;
  cout << a[0] - diff.begin()->first << " ";
  if (ok) {
    cout << a[0] + (a[1] - a[0]) / 2 << " ";
  }
  cout << a[n - 1] + diff.begin()->first << " ";
  return;
}
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(NULL);
  cout.tie(NULL);
  solve();
}
