#include <bits/stdc++.h>
using namespace std;
const long long mod = 1e9 + 7;
void solve() {
  ios_base ::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long n;
  cin >> n;
  string l, r;
  cin >> l >> r;
  vector<vector<long long>> left(26);
  vector<vector<long long>> right(26);
  vector<long long> ul;
  vector<long long> ur;
  vector<long long> mxl(26);
  vector<long long> mxr(26);
  for (long long i = 0; i < l.length(); i++) {
    if (l[i] != '?') {
      left[l[i] - 'a'].push_back(i);
    } else {
      ul.push_back(i);
    }
  }
  for (long long i = 0; i < r.length(); i++) {
    if (r[i] != '?') {
      right[r[i] - 'a'].push_back(i);
    } else {
      ur.push_back(i);
    }
  }
  vector<pair<long long, long long>> ans;
  for (long long i = 0; i < 26; i++) {
    for (long long j = 0; j < min(left[i].size(), right[i].size()); j++) {
      ans.push_back({left[i][j] + 1, right[i][j] + 1});
    }
    mxl[i] = mxr[i] = min(left[i].size(), right[i].size());
  }
  for (long long i = 0; i < 26; i++) {
    while (mxl[i] < left[i].size() && ur.size() > 0) {
      ans.push_back({left[i][mxl[i]] + 1, ur.back() + 1});
      ur.pop_back();
      mxl[i]++;
    }
  }
  for (long long i = 0; i < 26; i++) {
    while (mxr[i] < right[i].size() && ul.size() > 0) {
      ans.push_back({ul.back() + 1, right[i][mxr[i]] + 1});
      ul.pop_back();
      mxr[i]++;
    }
  }
  for (long long i = 0; i < min(ul.size(), ur.size()); i++) {
    ans.push_back({ul[i] + 1, ur[i] + 1});
  }
  cout << ans.size() << endl;
  for (long long i = 0; i < ans.size(); i++) {
    cout << ans[i].first << " " << ans[i].second << endl;
  }
}
int32_t main() {
  ios_base ::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long t = 1;
  for (long long i = 1; i <= t; i++) {
    solve();
  }
  return 0;
}
