#include <bits/stdc++.h>

using namespace std;

typedef long long LL;

map<char, queue<int> > M;

vector<pair<int, int> > res;

queue<int> other;

int main()

{

    int n;

    cin >> n;

    string a, b;

    cin >> a;

    for (int i = 0;i < n;i++)

        M[a[i]].push(i + 1);

    cin >> b;

    for (int i = 0;i < n;i++)

    {

        if (b[i] != '?')

        {

            if (M[b[i]].size() > 0)

            {

                res.push_back(make_pair(M[b[i]].front(), i + 1));

                M[b[i]].pop();

            }

            else if (M['?'].size() > 0)

            {

                res.push_back(make_pair(M['?'].front(), i + 1));

                M['?'].pop();

            }

        }

        else

            other.push(i + 1);

    }

    for (int i = 0;i <= 25;i++)

    {

        while (M[(char)(i + 'a')].size() && other.size())

        {

            res.push_back(make_pair(M[(char)(i + 'a')].front(), other.front()));

            M[(char)(i + 'a')].pop();

            other.pop();

        }

    }

    while (M['?'].size() && other.size())

    {

        res.push_back(make_pair(M['?'].front(), other.front()));

        M['?'].pop();

        other.pop();

    }

    cout << res.size() << endl;

    for (auto x : res)

        cout << x.first << ' ' << x.second << endl;

    return 0;

}