#include <bits/stdc++.h>   using namespace std;   const int MAXN = 1e6 + 5;   const long long MOD = 1e9 + 7;   set<char> seen;   void solve() {     int n;     cin >> n;     string a;     string b;     cin >> a >> b;     for (int i = 0; i < n; i++) {       if (a[i] > b[i]) {         cout <<  -1 n ;         return;       }     }     int ans = 0;     for (char c =  a ; c <=  t ; c++) {       char smallest =  z ;       for (int i = 0; i < n; i++) {         if (a[i] == c) {           if (a[i] != b[i]) smallest = min(smallest, b[i]);         }       }       if (smallest !=  z ) {         ans++;         for (int i = 0; i < n; i++) {           if (a[i] == c) {             if (a[i] != b[i]) {               a[i] = smallest;             }           }         }       }     }     cout << ans <<   n ;   }   int main() {     cin.sync_with_stdio(0);     cin.tie(0);     int TC;     cin >> TC;     while (TC--) {       solve();     }   }   