#include <bits/stdc++.h>
using namespace std;
const int inf = 0x3f3f3f3f;
const long long infl = 0x3f3f3f3f3f3f3f3f;
struct dsu {
  int _n;
  std::vector<int> parent_or_size;
  dsu() : _n(0) {}
  dsu(int n) : _n(n), parent_or_size(n, -1) {}
  int merge(int a, int b) {
    assert(0 <= a && a < _n);
    assert(0 <= b && b < _n);
    int x = leader(a), y = leader(b);
    if (x == y) return x;
    if (-parent_or_size[x] < -parent_or_size[y]) std::swap(x, y);
    parent_or_size[x] += parent_or_size[y];
    parent_or_size[y] = x;
    return x;
  }
  bool same(int a, int b) {
    assert(0 <= a && a < _n);
    assert(0 <= b && b < _n);
    return leader(a) == leader(b);
  }
  int leader(int a) {
    assert(0 <= a && a < _n);
    if (parent_or_size[a] < 0) return a;
    return parent_or_size[a] = leader(parent_or_size[a]);
  }
  int size(int a) {
    assert(0 <= a && a < _n);
    return -parent_or_size[leader(a)];
  }
  std::vector<std::vector<int>> groups() {
    std::vector<int> leader_buf(_n), group_size(_n);
    for (int i = 0; i < _n; i++) {
      leader_buf[i] = leader(i);
      group_size[leader_buf[i]]++;
    }
    std::vector<std::vector<int>> result(_n);
    for (int i = 0; i < _n; i++) {
      result[i].reserve(group_size[i]);
    }
    for (int i = 0; i < _n; i++) {
      result[leader_buf[i]].push_back(i);
    }
    result.erase(
        std::remove_if(result.begin(), result.end(),
                       [&](const std::vector<int>& v) { return v.empty(); }),
        result.end());
    return result;
  }
};
void solve() {
  int n;
  cin >> n;
  string a, b;
  cin >> a >> b;
  for (int i = 0; i < (n); i++)
    if (b[i] < a[i]) {
      cout << -1 << '\n';
      return;
    }
  dsu d(20);
  int ans = 0;
  for (int i = 0; i < (n); i++) {
    int aa = a[i] - 'a', bb = b[i] - 'a';
    if (d.same(aa, bb)) continue;
    ans++;
    d.merge(aa, bb);
  }
  cout << ans << '\n';
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int tc = 1;
  cin >> tc;
  while (tc--) solve();
  return 0;
}
