func_code_string: #include <bits/stdc++.h>   using namespace std;   void solve() {     int N;     cin >> N;     string s1, s2;     cin >> s1 >> s2;     int num_different = 0;     vector<pair<bool, int>> differences;     int num_s1_has_a = 0;     int num_s1_has_b = 0;     for (int i = 0; i < N; ++i) {       if (s1[i] != s2[i]) {         bool s1_has_a = (s1[i] ==  a );         if (s1_has_a) {           ++num_s1_has_a;         } else {           ++num_s1_has_b;         }         differences.push_back({s1_has_a, i});       }     }     if ((num_s1_has_a + num_s1_has_b) % 2 != 0) {       cout << -1 << endl;       return;     } else {       sort(differences.begin(), differences.end());       int answer = (num_s1_has_a + num_s1_has_b) / 2;       if (num_s1_has_a % 2 != 0) {         ++answer;       }       cout << answer << endl;       for (int i = 0; i < differences.size(); i += 2) {         if (differences[i].first != differences[i + 1].first) {           cout << differences[i].second + 1 <<     << differences[i].second + 1                << endl;         }         cout << differences[i].second + 1 <<     << differences[i + 1].second + 1              << endl;       }     }   }   int main() { solve(); }   