#include <bits/stdc++.h>
using namespace std;

/* Madara Uchiha */
#define TRACE

#ifdef TRACE
#define dbgarr(a,n)   cerr << "["; for(int i = 0; i < n; ++i) cerr << a[i] << " ";cerr << "\b]\n";
#define dbg(args...)  {debug,args; cerr<<endl;}
#define pause()       cin.get();cin.get();

#else
#define dbgarr(a,n)
#define dbg(args...)
#define pause()

#endif

struct debugger {
    template<typename T> debugger& operator , (const T& v) {
        cerr<<v<<" "; return *this;
    }
} debug;

template <typename T1, typename T2>
inline ostream& operator << (ostream& os, const pair<T1, T2>& p) {
    return os << "(" << p.first << ", " << p.second << ")";
}

template<typename T>
inline ostream &operator << (ostream & os,const vector<T>& v) {
    bool first = true; os << "[";
    for (typename vector<T>::const_iterator ii = v.begin(); ii != v.end(); ++ii) {
        if(!first) os << ", ";
        os << *ii; first = false;
    }
    return os << "]";
}

#define each(i,c) for(__typeof((c).begin()) i=(c).begin();i!=(c).end();i++)
#define fr first
#define se second
typedef long long LL;
typedef pair<int,int> pii;
typedef vector<int> vi;

// for each i find (a,b,c,d) ,such that d = i and a is max
const int NN = (1 << 20);

int match[NN], seg[2*NN];
int inp[NN], od[NN];
int nxt[NN], prev[NN];

void update(int node,int st,int en,int ind){
    if (st == en){
        seg[node] = nxt[ind];
        return;
    }

    int mid = (st+en)/2;
    if (mid >= ind) update(2*node,st,mid,ind);
    else update(2*node+1,mid+1,en,ind);
    seg[node] = max(seg[2*node],seg[2*node+1]);
}

int query(int node,int st,int en,int L,int R,int rng){
    if (st > R or en < L or st > en) return -1;
    if (st >= L and en <= R){
        if (seg[node] <= rng) return -1;
        if (st == en) return st;
        int mid = (st+en)/2;
        if (seg[2*node+1] > rng)
            return query(2*node+1,mid+1,en,L,R,rng);
        return query(2*node,st,mid,L,R,rng);
    }

    int mid = (st+en)/2, ans = -1;
    if (mid >= L)
        ans = max(ans,query(2*node,st,mid,L,R,rng));
    if (mid+1 <= R)
        ans = max(ans,query(2*node+1,mid+1,en,L,R,rng));
    return ans;
}

bool cmp1(int x,int y){
    if (inp[x] == inp[y])
        return (x < y);
    return (inp[x] < inp[y]);
}

bool cmp2(int x,int y){
    if (nxt[x] == nxt[y])
        return (x < y);
    return (nxt[x] < nxt[y]);
}

void solve(){
    int n;
    cin >> n;

    memset(seg,-1,sizeof(seg));
    for(int i = 0; i < n; ++i){
        cin >> inp[i], od[i] = i;
        nxt[i] = n+5, prev[i] = -2;
        match[i] = -1;
    }

    sort(od,od+n,cmp1);
    for(int i = 0; i < n; ++i){
        int x = od[i];
        if (i+1 < n and inp[od[i+1]] == inp[x] and od[i+1] > x+1)
            nxt[x] = od[i+1];
        else if (i+2 < n and inp[od[i+2]] == inp[x])
            nxt[x] = od[i+2];
        if (i-1 >= 0 and inp[od[i-1]] == inp[x] and od[i-1] < x-1)
            prev[x] = od[i-1];
        else if (i-2 >= 0 and inp[od[i-2]] == inp[x])
            prev[x] = od[i-2];

        assert(nxt[i] > i+1);
        assert(prev[i] < i-1);
    }

    for(int i = 0; i < n; ++i)
        od[i] = i;

    sort(od,od+n,cmp2);
    for(int i = 0, j = 0; i < n; ++i){
        while(j < n and nxt[od[j]] < i)
            update(1,0,n-1,od[j]), ++j;

        int x = prev[i];
        if (x <= 0) continue;
        match[i] = query(1,0,n-1,0,x-1,x);
    }

    vi ans;
    for(int i = 0, k = 0; i < n; ++i){
        if (match[i] >= k){
            ans.push_back(match[i]);
            ans.push_back(i);
            ans.push_back(match[i]);
            ans.push_back(i);
            k = i+1;
        }
    }

    int m = (int)ans.size();
    cout << m << endl;
    for(int i = 0; i < m; ++i)
        cout << inp[ans[i]] << " ";
    cout << endl;
}

int main()
{
    ios_base::sync_with_stdio(0);
    solve();
    return 0;
}