#include <bits/stdc++.h>
using namespace std;
inline bool isvowel(char c) {
  c = tolower(c);
  if (c == 'a' || c == 'e' || c == 'i' || c == 'y' || c == 'o' || c == 'u')
    return 1;
  return 0;
}
const double eps = 0.000001;
const long double pi = acos(-1);
const int maxn = 1e7 + 9;
const int mod = 1e9 + 7;
const long long MOD = 1e18 + 9;
const long long INF = 1e18 + 123;
const int inf = 2e9 + 11;
const int mxn = 1e6 + 9;
const int N = 6e5 + 123;
const int M = 22;
const int pri = 997;
const int Magic = 2101;
const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, -1, 0, 1};
int n, m, k;
vector<int> ans;
int len;
int a[N];
pair<int, int> st[N];
map<int, int> cnt, L;
void add(int a, int b) {
  ans.push_back(a);
  ans.push_back(b);
  ans.push_back(a);
  ans.push_back(b);
  len = 0;
  cnt.clear();
  L.clear();
}
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  for (int i = 1; i <= n; i++) {
    cnt[a[i]]++;
    if (cnt[a[i]] == 4) {
      add(a[i], a[i]);
      continue;
    }
    if (!L.count(a[i])) L[a[i]] = i;
    while (len > 0 && L[st[len].second] > L[a[i]]) {
      len--;
    }
    if (st[len].first > L[a[i]] && L[st[len].second] < L[a[i]]) {
      add(st[len].second, a[i]);
      continue;
    }
    st[++len] = make_pair(i, a[i]);
  }
  cout << ans.size() << '\n';
  for (auto to : ans) cout << to << ' ';
  return 0;
}
