#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const double eps = 1e-9;
const int maxn = 100000 + 100;
int a[maxn], b[maxn], ans[maxn], cnt;
int main() {
  int T, cas = 1;
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; ++i) scanf("%d", &a[i]);
  sort(a, a + n);
  if (n == 1) {
    puts("-1");
    return 0;
  } else if (n == 2 && a[0] == a[1]) {
    puts("1");
    printf("%d\n", a[0]);
    return 0;
  } else if (n == 2) {
    ans[cnt++] = a[0] - (a[1] - a[0]);
    if ((a[1] - a[0]) % 2 == 0) ans[cnt++] = (a[1] + a[0]) / 2;
    ans[cnt++] = a[1] + (a[1] - a[0]);
  } else {
    for (int i = 1; i < n; ++i) b[i - 1] = a[i] - a[i - 1];
    int bn = n - 1;
    sort(b, b + bn);
    bn = unique(b, b + bn) - b;
    int f = 1;
    if (b[0] == 0 && bn == 1) {
      puts("1");
      printf("%d\n", a[0]);
      return 0;
    } else if (b[0] == 0)
      f = 0;
    for (int i = 1; i < bn && f; ++i) {
      if (b[i] % b[0]) f = 0;
    }
    if (!f) {
      puts("0");
      return 0;
    } else {
      if (bn == 1) {
        ans[cnt++] = a[0] - b[0];
        ans[cnt++] = a[n - 1] + b[0];
      } else {
        int t = a[0] + b[0];
        for (int i = 1; i < n && f; ++i) {
          while (a[i] != t && f) {
            if (f == 1)
              ans[cnt++] = t, f++;
            else if (f == 2)
              f = 0;
            t += b[0];
          }
          t += b[0];
        }
        if (!f) {
          puts("0");
          return 0;
        }
      }
    }
  }
  printf("%d\n", cnt);
  for (int i = 0; i < cnt; ++i) {
    printf("%d%c", ans[i], i == cnt - 1 ? '\n' : ' ');
  }
  return 0;
}
