func_code_string: #include <bits/stdc++.h>   using namespace std;   int w;   string ch(string s, char x, string b) {     int n = s.size();     bool cx = 0;     char mina =  z ;     for (int i = 0; i < n; i++) {       if (s[i] == x && b[i] > s[i]) {         cx = 1;         mina = min(mina, b[i]);       }     }     if (!cx) {       return s;     }     w++;     for (int i = 0; i < n; i++) {       if (s[i] == x) s[i] = mina;     }     return s;   }   void solve() {     w = 0;     int n;     cin >> n;     string a, b;     cin >> a >> b;     for (int i = 0; i < n; i++) {       if (b[i] < a[i]) {         cout <<  -1 n ;         return;       }     }     for (int i = 0; i < 20; i++) {       a = ch(a, ((char)i + 97), b);     }     cout << w <<   n ;   }   int main() {     ios_base::sync_with_stdio(0);     cin.tie(0);     cout.tie(0);     int t;     cin >> t;     while (t) {       t--;       solve();     }   }   