#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1e9 + 7;
const long long N = 2e5 + 5;
const double pi = acos(-1.0);
long long addM(long long a, long long b) { return (a + b) % MOD; }
long long multM(long long a, long long b) { return (a * b) % MOD; }
long long binpow(long long a, long long b) {
  long long res = 1;
  while (b > 0) {
    if (b & 1) res *= a;
    a *= a;
    b >>= 1;
  }
  return res;
}
long long ModInverse(long long n) { return binpow(n, MOD - 2); }
vector<long long> findFact() {
  vector<long long> fact(N);
  fact[0] = 1;
  for (long long i = 1; i <= N; i++) {
    fact[i] = multM(i, fact[i - 1]);
  }
  return fact;
}
long long ncr(long long n, long long r) {
  vector<long long> fact = findFact();
  return ((fact[n] * ModInverse(fact[n - r])) % MOD * ModInverse(fact[r])) %
         MOD;
}
vector<bool> findPrime() {
  vector<bool> is_prime(N + 1, true);
  is_prime[0] = false;
  is_prime[1] = false;
  for (long long i = 2; i <= N; i++) {
    if (is_prime[i] && i * i <= N) {
      for (long long j = i * i; j <= N; j += i) is_prime[j] = false;
    }
  }
  return is_prime;
}
vector<bool> segmentedSieveNoPreGen(long long L, long long R) {
  vector<bool> isPrime(R - L + 1, true);
  long long lim = sqrt(R);
  for (long long i = 2; i <= lim; ++i)
    for (long long j = max(i * i, (L + i - 1) / i * i); j <= R; j += i)
      isPrime[j - L] = false;
  if (L == 1) isPrime[0] = false;
  return isPrime;
}
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long tt;
  cin >> tt;
  while (tt--) {
    long long n;
    cin >> n;
    string x, y;
    cin >> x >> y;
    bool check = true;
    for (long long i = 0; i < n; i++) {
      if (x[i] > y[i]) {
        check = false;
      }
    }
    if (check) {
      long long ans = 0;
      for (char c = 'a'; c <= 'z'; c++) {
        char mn = 'z';
        bool check = false;
        for (long long i = 0; i < n; i++) {
          if (x[i] == c && x[i] != y[i]) {
            check = true;
            mn = min(mn, y[i]);
          }
        }
        for (long long i = 0; i < n; i++) {
          if (x[i] == c && x[i] != y[i]) {
            x[i] = mn;
          }
        }
        if (check) ans++;
      }
      cout << ans << "\n";
    } else {
      cout << "-1\n";
    }
  }
  return 0;
}
