#include <bits/stdc++.h>
using namespace std;
const int INF = (1 << 29);
const int INFF = 0x7fffffff;
const long long LINF = (1ll << 60);
const long long LINFF = 0x7fffffffffffffff;
const long double PI = 3.14159265359;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
long long rnd(long long a, long long b) {
  return uniform_int_distribution<long long>(a, b)(rng);
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int t;
  cin >> t;
  while (t--) {
    int n;
    cin >> n;
    string a, b;
    cin >> a >> b;
    bool ans = true;
    bool change[300][300] = {};
    for (int i = 0; i < n; i++) {
      if (a[i] != b[i] && a[i] > b[i]) ans = false;
      if (a[i] != b[i]) change[a[i]][b[i]] = true;
    }
    int cnt = 0;
    for (int c = 'a'; c <= 't'; c++) {
      for (int d = c + 1; d <= 't'; d++) {
        if (change[c][d] == 0) continue;
        cnt += change[c][d];
        for (int e = d + 1; e <= 't'; e++) {
          change[d][e] += change[c][e];
        }
        break;
      }
    }
    if (ans == false) cnt = -1;
    cout << cnt << '\n';
  }
  return 0;
}
