#include <bits/stdc++.h>
#pragma warning(disable : 4996)
using namespace std;
void open() {
  ((void)0);
  ((void)0);
  ((void)0);
}
const int MAX = 1.5e5;
const int MOD = 1e9 + 7;
const int INF = 0x3f3f3f3f;
const long long LL_INF = 0x3f3f3f3f3f3f3f3fLL;
const double PI = acos(-1);
const long double ERR = 1e-10;
const int move_r[] = {-1, 0, 1, 0, -1, -1, 1, 1};
const int move_c[] = {0, -1, 0, 1, -1, 1, -1, 1};
int N;
char L[MAX + 1], R[MAX + 1];
vector<int> idxL[27], idxR[27];
vector<pair<int, int> > ans;
int main(int argc, char *argv[]) {
  open();
  scanf("%d %s %s", &N, L, R);
  for (int i = 0; L[i]; ++i) {
    if (L[i] == '?')
      idxL[26].push_back(i + 1);
    else
      idxL[L[i] - 'a'].push_back(i + 1);
    if (R[i] == '?')
      idxR[26].push_back(i + 1);
    else
      idxR[R[i] - 'a'].push_back(i + 1);
  }
  for (int i = 0; i < 27; ++i) {
    while (!idxL[i].empty() && !idxR[i].empty()) {
      ans.push_back({idxL[i].back(), idxR[i].back()});
      idxL[i].pop_back();
      idxR[i].pop_back();
    }
    while (!idxL[i].empty() & !idxR[26].empty()) {
      ans.push_back({idxL[i].back(), idxR[26].back()});
      idxL[i].pop_back();
      idxR[26].pop_back();
    }
    while (!idxL[26].empty() & !idxR[i].empty()) {
      ans.push_back({idxL[26].back(), idxR[i].back()});
      idxL[26].pop_back();
      idxR[i].pop_back();
    }
  }
  printf("%lu\n", ans.size());
  for (pair<int, int> p : ans) printf("%d %d\n", p.first, p.second);
  return 0;
}
