#include <iostream>
#include <queue>
#include <bits/stdc++.h>

using namespace std;

const int N = 150000 + 100;

map<char, queue<int>> cnt1, cnt2;
int n;
char l[N], r[N];

inline void read(){
    cin >> n;
    for(int i = 0; i < n; ++i){
        cin >> l[i];
        cnt1[l[i]].push(i);
    }
    for(int i = 0; i < n; ++i){
        cin >> r[i];
        cnt2[r[i]].push(i);
    }
}

int main(void){
    read();
    vector<pair<int, int>> ans;
    for(char i = 'a'; i <= 'z'; ++i){
        if(!cnt1[i].empty() && !cnt2[i].empty()){
            while(!cnt1[i].empty() && !cnt2[i].empty()){
                int x = cnt1[i].front(), y = cnt2[i].front();
                cnt1[i].pop(), cnt2[i].pop();
                ans.push_back(make_pair(x + 1, y + 1));
                if(cnt2[i].empty() || cnt1[i].empty()){
                    break;
                }
            }
        }
    }
    if(!cnt1['?'].empty()){
        for(char i = 'a'; i <= 'z'; ++i){
            if(!cnt1['?'].empty() && !cnt2[i].empty()){
                while(!cnt1['?'].empty() && !cnt2[i].empty()){
                    int x = cnt1['?'].front(), y = cnt2[i].front();
                    cnt1['?'].pop(), cnt2[i].pop();
                    ans.push_back(make_pair(x + 1, y + 1));
                    if(cnt2[i].empty() || cnt1['?'].empty()){
                        break;
                    }
                }
            }
        }
    }
    if(!cnt2['?'].empty()){
        for(char i = 'a'; i <= 'z'; ++i){
            if(!cnt2['?'].empty() && !cnt1[i].empty()){
                while(!cnt2['?'].empty() && !cnt1[i].empty()){
                    int y = cnt2['?'].front(), x = cnt1[i].front();
                    cnt2['?'].pop(), cnt1[i].pop();
                    ans.push_back(make_pair(x + 1, y + 1));
                    if(cnt1[i].empty() || cnt2['?'].empty()){
                        break;
                    }
                }
            }
        }
    }
    queue<int> q1 = cnt1['?'];
    queue<int> q2 = cnt2['?'];
    if(!q1.empty() && !q2.empty()){
                while(!q1.empty() && !q2.empty()){
                    int x = q1.front(), y = q2.front();
                    q1.pop(), q2.pop();
                    ans.push_back(make_pair(x + 1, y + 1));
                    if(q2.empty() || q1.empty()){
                        break;
                    }
                }
            }
    cout << ans.size() << endl;
    for(int i = 0; i < ans.size(); ++i){
        cout << ans[i].first << " " << ans[i].second << endl;
    }
    return 0;
}