#include <bits/stdc++.h>   using namespace std;   int main() {     int t;     cin >> t;     while (t--) {       int n;       cin >> n;       string s1, s2;       cin >> s1 >> s2;       bool poss = true;       int change[20][20];       memset(change, 0, sizeof(change));       for (int i = 0; i < n; i++) {         if (s1[i] > s2[i])           poss = false;         else {           if (s1[i] != s2[i]) {             change[s1[i] - 97][s2[i] - 97]++;           }         }       }       if (!poss) {         cout << -1 << endl;         continue;       }       int steps = 0;       for (int i = 0; i < 19; i++) {         int x = -1;         for (int j = 1; j < 20; j++) {           if (change[i][j] > 0) {             steps++;             x = j;             break;           }         }         if (x != -1) {           for (int j = x + 1; j <= 20; j++) {             change[x][j] += change[i][j];           }         }       }       cout << steps << endl;     }   }   