func_code_string: #include <bits/stdc++.h>   using namespace std;   int c[500];   int main() {     string s, t;     int n, i, j, k;     cin >> s >> t;     bool ok = 1;     vector<pair<char, char> > ans;     n = s.length();     vector<char> v(500,  0 );     for (i = 0; i < n; i++) {       if (s[i] == t[i]) {         if (c[s[i]] == -2) ok = 0;         c[s[i]] = -1;         continue;       } else if (c[s[i]] == -1 || c[t[i]] == -1) {         ok = 0;       } else if (!c[s[i]] && !c[t[i]]) {         ans.push_back({s[i], t[i]});         c[s[i]] = -2;         v[s[i]] = t[i];         c[t[i]] = -2;         v[t[i]] = s[i];       } else if (c[s[i]] == -2 && c[t[i]] == -2) {         if (v[s[i]] != t[i] || v[t[i]] != s[i]) ok = 0;       } else         ok = 0;     }     if (!ok) return cout << -1, 0;     cout << ans.size() << endl;     for (i = 0; i < ans.size(); i++) {       cout << ans[i].first <<     << ans[i].second << endl;     }   }   