#include <bits/stdc++.h>   using namespace std;   const long long base = 7;   const long long maxn = 1e6 + 9;   const long long inf = 1e9 + 7;   queue<long long> l[30], r[30], l1, r1, lf, rf;   vector<pair<long long, long long> > res;   long long n, i;   string a, b;   int main() {     ios_base::sync_with_stdio(false);     cin.tie(0);     cout.tie(0);     cin >> n >> a >> b;     a =     + a;     b =     + b;     for (i = 1; i <= n; i++) {       if (a[i] !=  ? )         l[a[i] -  a ].push(i);       else         l1.push(i);     }     for (i = 1; i <= n; i++) {       if (b[i] !=  ? )         r[b[i] -  a ].push(i);       else         r1.push(i);     }     for (i = 0; i <= 27; i++) {       while (true) {         if (!l[i].empty() && !r[i].empty()) {           res.push_back(make_pair(l[i].front(), r[i].front()));           l[i].pop();           r[i].pop();         } else {           while (!l[i].empty()) {             lf.push(l[i].front());             l[i].pop();           }           while (!r[i].empty()) {             rf.push(r[i].front());             r[i].pop();           }           break;         }       }     }     while (true) {       if (!lf.empty() && !r1.empty()) {         res.push_back(make_pair(lf.front(), r1.front()));         lf.pop();         r1.pop();       } else         break;     }     while (true) {       if (!rf.empty() && !l1.empty()) {         res.push_back(make_pair(l1.front(), rf.front()));         rf.pop();         l1.pop();       } else         break;     }     while (true) {       if (!r1.empty() && !l1.empty()) {         res.push_back(make_pair(l1.front(), r1.front()));         r1.pop();         l1.pop();       } else         break;     }     cout << res.size() <<   n ;     for (i = 0; i < res.size(); i++)       cout << res[i].first <<     << res[i].second <<   n ;   }   