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 os
import sys
from collections import Counter
from math import log2
from io import BytesIO, IOBase
nmbr = lambda: int(input())
lst = lambda: list(map(int, input().split()))
def main():
    for _ in range(nmbr()):  # zzzzz
        n, k = lst()
        s = input()
        mp = {}
        p = -1
        for i in range(n):
            # change s[i] to s[i]+1 'a' -> 'b'  , see for 'z'
            if s[i] == 'z': continue
            changed = chr(ord(s[i]) + 1)
            mp[changed] = mp.get(changed, 0) + 1
            needed = 0
            for ch, freq in mp.items():
                if freq == 0: continue
                needed += k - freq % k - (k if freq % k == 0 else 0)
            left = n - (i + 1) - needed
            if left < 0 or left % k != 0: continue
            p = i
            mp[changed] -= 1
            mp[s[i]] = mp.get(s[i], 0) + 1
        f = 1
        for k1, v in mp.items():
            if v % k != 0:
                f = 0
                break
        if p == -1:
            if f:
                print(s)
            else:
                print(-1)
            continue
        # print(p)
        mp = {}
        for ch in s[:p]:
            mp[ch] = mp.get(ch, 0) + 1
        changed = chr(ord(s[p]) + 1)
        mp[changed] = mp.get(changed, 0) + 1
        needed = 0
        xx = []
        for ch, freq in mp.items():
            if freq == 0: continue
            times = k - freq % k - (k if freq % k == 0 else 0)
            needed += times
            xx += [ch] * times
        left = n - (p + 1) - needed
        ans = []
        for i in range(p):
            ans += [s[i]]
        ans += [changed]
        for i in range(left):
            ans += ['a']
        xx.sort()
        ans += xx
        ans = ''.join(ans)
        # print(ans)
        if f:
            ans=min(ans, s)
        sys.stdout.write(ans+'\n')
    # sys.stdout.write(str(2*ans)+'\n')
BUFSIZE = 8192

class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None

    def read(self):
        while True:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            if not b:
                break
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines = 0
        return self.buffer.read()

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0), self.buffer.seek(0)


class IOWrapper(IOBase):
    def __init__(self, file):
        self.buffer = FastIO(file)
        self.flush = self.buffer.flush
        self.writable = self.buffer.writable
        self.write = lambda s: self.buffer.write(s.encode("ascii"))
        self.read = lambda: self.buffer.read().decode("ascii")
        self.readline = lambda: self.buffer.readline().decode("ascii")


sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)
input = lambda: sys.stdin.readline().rstrip("\r\n")

if __name__ == "__main__":
    for t in range(1):main()#int(input())):