#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t, n;
  cin >> t;
  while (t--) {
    string A, B;
    cin >> n;
    cin >> A >> B;
    int f = 1;
    for (int i = 0; i < n; ++i) {
      if (A[i] > B[i]) f = 0;
    }
    if (!f) {
      cout << -1 << '\n';
      continue;
    }
    int ans = 0;
    for (int i = 0; i < 26; ++i) {
      bitset<27> good;
      for (int j = 0; j < n; ++j) {
        if (A[j] == B[j]) continue;
        if (B[j] == char('a' + i)) {
          good[A[j] - 'a'] = 1;
        }
      }
      ans += good.count();
      for (int j = 0; j < n; ++j) {
        if (A[j] == B[j]) continue;
        if (good[A[j] - 'a'] == 1) A[j] = char('a' + i);
      }
    }
    cout << ans << '\n';
  }
  return 0;
}
