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

int main() {
    int n; cin >> n;
    vector<int> G(n+1,0);
    vector<int> van(n+1,0);
    queue<int> a;
    queue<int> b;
    for(int i=1;i<=n;i++){
        cin >> G[i];
        van[G[i]]++;
        if(van[G[i]]>1) b.push(i);
    }
    for(int i=1;i<=n;i++){
        if(van[i]==0) a.push(i);    
    }

    cout << a.size() << endl;
    while(!a.empty()){
        cout << b.front() << " " << a.front() << endl;
        a.pop();
        b.pop();
    }
}