#include <bits/stdc++.h>
using namespace std;
const int N = 200005;
int n;
string a, b;
vector<int> g1[26], g2[26];
vector<int> a1, a2;
vector<pair<int, int>> ans;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cin >> n;
  cin >> a;
  cin >> b;
  for (int i = 0; i < n; i++) {
    if (a[i] == '?') {
      a1.push_back(i);
      continue;
    }
    int d = a[i] - 'a';
    g1[d].push_back(i);
  }
  for (int i = 0; i < n; i++) {
    if (b[i] == '?') {
      a2.push_back(i);
      continue;
    }
    int d = b[i] - 'a';
    g2[d].push_back(i);
  }
  for (int i = 0; i < 26; i++) {
    while (!g1[i].empty() and !g2[i].empty()) {
      ans.push_back(make_pair(g1[i].back(), g2[i].back()));
      g1[i].pop_back();
      g2[i].pop_back();
    }
  }
  for (int i = 0; i < 26; i++) {
    while (!g1[i].empty() and !a2.empty()) {
      ans.push_back(make_pair(g1[i].back(), a2.back()));
      g1[i].pop_back();
      a2.pop_back();
    }
  }
  for (int i = 0; i < 26; i++) {
    while (!g2[i].empty() and !a1.empty()) {
      ans.push_back(make_pair(a1.back(), g2[i].back()));
      g2[i].pop_back();
      a1.pop_back();
    }
  }
  while (!a1.empty() and !a2.empty()) {
    ans.push_back(make_pair(a1.back(), a2.back()));
    a1.pop_back();
    a2.pop_back();
  }
  cout << ans.size() << endl;
  for (int i = 0; i < (int)ans.size(); i++) {
    cout << ans[i].first + 1 << " " << ans[i].second + 1 << "\n";
  }
  return 0;
}
