#include <iostream>
#include <map>
#include <vector>
#include <algorithm>
using namespace std;

bool pComp(pair<int, int> a, pair<int, int> b) {
    if (a.first > b.first) return true;
    else if (a.first == b.first) {
        if (a.second > b.second) return true;
        else return false;
    }
    else return false;
}

int main()
{
    int n;
    cin >> n;
    map<int, int> change;
    vector<int> inp(n);
    vector<pair<int, int>> updates;
    for (size_t i = 0; i < n; i++)
    {
        cin >> inp[i];
    }
    for (size_t i = 0; i < n/2; i++)
    {
        int one = inp[i];
        int two = inp[n - i - 1];
        if (change[one] != 0) {
            one = change[one];
        }
        if (change[two] != 0) {
            two = change[two];
        }
        if (one == two) continue;
        else {
            int mx = max(one, two);
            int mn = min(one, two);
            updates.push_back({ mx, mn });
            change[mx] = mn;
        }
    }
    sort(updates.begin(), updates.end(), pComp);
    cout << updates.size() << "\n";
    for (size_t i = 0; i < updates.size(); i++)
    {
        cout << updates[i].first << " " << updates[i].second << "\n";
    }
}