tempCodeRunnerFile.python

ALPHABET = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
t = int(input())
for test_case in range(t):
    before = input()
    after = input()
 
    becomes = {}
    possible = True
    for i in range(len(before)):
        if before[i] in becomes and becomes[before[i]] != after[i]:
            possible = False
        becomes[before[i]] = after[i]
    if len(set(after)) == 52 and before != after:
        possible = False
 
    answer = 0
    if possible:
        in_degree = {}
        for r in ALPHABET:
            if r in becomes and becomes[r] != r:
                in_degree[becomes[r]] = in_degree.get(becomes[r], 0) + 1
                answer += 1
        seen = {}
        for r in ALPHABET:
            if r not in seen:
                a = r
                while a not in seen:
                    seen[a] = r
                    a = becomes.get(a, a)
                if a in becomes and a != becomes[a] and seen[a] == r:
                    s = a
                    cycle = True
                    while True:
                        seen[a] = 'moo'
                        if in_degree.get(a, 0) > 1:
                            cycle = False
                        a = becomes[a]
                        if a == s:
                            break
                    if cycle:
                        answer += 1
        print(answer)
    else:
        print(-1)