12
12 3
zazasqzqsasq
1 1
z
1 1
a
15 4
abccbaabccbazaa
16 8
cadjfdsljfdkljds
6 2
baazaa
6 3
bosszb
6 1
qwerty
7 7
daaaaaa
14 2
acabcbdeffewzd
18 6
zzzzzzzzzzzzyyyyyz
9 4
aabaabbbc

zazasqzqsasq
z
a
-1
cbbbbbbbbccccccc
baazbz
bpbbpp
qwerty
ddddddd
acabcbdeffexdx
zzzzzzzzzzzzzzzzzz
-1

import math
from collections import defaultdict, Counter
from functools import lru_cache

t = int(input())
for _ in range(t):
    n, k = [int(i) for i in input().split()]
    alpha = [chr(x) for x in range(ord('a'), ord('z') + 1)]
    s = input()
    count = Counter(s)
    if n % k != 0:
        print(-1)
    else:
        keys = list(count.keys())
        nb = 0
        for i in keys:
            if count[i] % k != 0:
                nb = 1
                break
        if nb == 0:
            print(s)
        else:
            flag = 0
            ans = 'z' * n
            avail = 0
            for i in range(n-1, -1, -1):
                count[s[i]] -= 1
                if s[i] == 'z':
                    avail += 1
                    continue
                else:
                    for ele in alpha:
                        if ele <= s[i]:
                            continue
                        count[ele] += 1
                        temp = 0
                        avail1 = avail
                        s2 = ele
                        for char in alpha:
                            if count[char] % k != 0:
                                req = (k - (count[char] % k))
                                if req > avail1:
                                    temp = 1
                                    break
                                else:
                                    s2 += (req * char)
                                    avail1 -= req
                                if temp == 1:
                                    break
                        count[ele] -= 1
                        if temp == 0:
                            flag = 1
                            ans = s[:i] + s2
                            break
                    if flag == 1:
                        break
                avail += 1
            print(ans)








