#include <bits/stdc++.h>
using namespace std;
using gg = long long;
constexpr gg MAX = 1e6 + 5;
constexpr gg mod = 1e9 + 7;
constexpr gg INF = 1e10;
constexpr double thre = 1e-7;
gg ti, ni, mi, ki, di, pi, xi, yi;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  ti = 1;
  cin >> ti;
  while (ti--) {
    string a, b;
    cin >> ni >> a >> b;
    map<char, set<char>> m;
    for (gg i = 0; i < ni; ++i) {
      if (a[i] != b[i]) {
        m[a[i]].insert(b[i]);
      }
    }
    gg ans = 0;
    bool f = true;
    while (not m.empty()) {
      auto& [i, j] = *m.begin();
      if (*j.begin() < i) {
        f = false;
        break;
      }
      if (j.size() != 1) {
        for (auto k = next(j.begin()); k != j.end(); ++k) {
          m[*j.begin()].insert(*k);
        }
      }
      m.erase(m.begin());
      ans++;
    }
    cout << (f ? ans : -1) << "\n";
  }
  return 0;
}
