string s, s1;
int p[26];
vector<pair <char, char> > ans;

int solve()
{
    forn(i, 26)
        p[i] = i;
    cin >> s >> s1;
    forn(i, s.size())
    {
        if (s[i] == s1[i])
        {
            if (p[s[i] - 'a'] != s[i] - 'a')
                puts("-1"), exit(0);
        }
        else
        {
            if (p[s[i] - 'a'] == s[i] - 'a' && p[s1[i] - 'a'] == s1[i] - 'a')
            {
                p[s[i] - 'a'] = s1[i] - 'a';
                p[s1[i] - 'a'] = s[i] - 'a';
                ans.inb(mk(s[i], s1[i]));
            }
            else
            {
                if (p[s[i] - 'a'] != s1[i] - 'a' || p[s1[i] - 'a'] != s[i] - 'a')
                    puts("-1"), exit(0);
            }
        }
    }
    cout << ans.size() << "\n";
    forn(i, ans.size())
        cout << ans[i].X << " " << ans[i].Y << "\n";
    return 0;
}