#include <bits/stdc++.h>
using namespace std;
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T>
void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x) cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cerr << ", ";
  _print(v...);
}
void pre() {}
bool fg[20][20];
int nxt[20];
void solve() {
  int n;
  cin >> n;
  string a, b;
  cin >> a >> b;
  bool ans = 0;
  memset(fg, 0, sizeof(fg));
  for (int i = 0; i < (n); ++i) {
    if (a[i] > b[i])
      ans = 1;
    else
      fg[a[i] - 'a'][b[i] - 'a'] = 1;
  }
  if (ans)
    cout << -1 << '\n';
  else {
    for (int i = 0; i < (20); ++i) nxt[i] = -1;
    int cnt = 0;
    for (int i = 0; i < (20); ++i) {
      int lst = -1;
      for (int j = i + 1; j <= (19); ++j) {
        if (fg[i][j]) {
          if (lst == -1)
            nxt[i] = j;
          else
            fg[lst][j] = 1;
          lst = j;
        }
      }
      if (nxt[i] != -1) cnt++;
    }
    cout << cnt << '\n';
  }
}
int main() {
  cin.sync_with_stdio(0);
  cin.tie(0);
  cin.exceptions(cin.failbit);
  pre();
  int n;
  cin >> n;
  for (int i = 0; i < (n); ++i) solve();
  return 0;
}
