func_code_string: #include <bits/stdc++.h>   using namespace std;   int main() {     string s, t;     cin >> s >> t;     int n = s.size();     char c;     vector<pair<char, char>> a;     for (int i = 0; i < n; i++)       if (s[i] != t[i]) {         c = t[i];         for (int j = 0; j < n; j++)           if (t[j] == c && s[j] == s[i] || t[j] == s[i] && s[j] == c) {             if (t[j] == c)               t[j] = s[i];             else               t[j] = c;           } else if (t[j] == c || s[j] == s[i] || t[j] == s[i] || s[j] == c) {             cout << -1;             return 0;           }         a.push_back(make_pair(c, s[i]));       }     cout << a.size() << endl;     for (int i = 0; i < a.size(); i++)       cout << a[i].first <<     << a[i].second << endl;     return 0;   }   