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 sys
import math
from collections import defaultdict,Counter,deque

# input=sys.stdin.readline
# def print(x):
#     sys.stdout.write(str(x)+"\n")

import os
import sys
from io import BytesIO, IOBase

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")

# sys.stdout=open("CP2/output.txt",'w')
# sys.stdin=open("CP2/input.txt",'r')

# mod=pow(10,9)+7
t=int(input())
for i in range(t):
	n,k=map(int,input().split())
	s=list(input())
	d=defaultdict(int)
	if n%k!=0:
		print(-1)
		continue
	for j in range(n):
		d[ord(s[j])]+=1
	cur=0
	for j in d:
		c=d[j]%k
		if c:
			d[j]=k-c
		else:
			d[j]=0
		cur+=d[j]
	if cur==0:
		print(''.join(s))
		continue
	for j in range(n-1,-1,-1):
		l=ord(s[j])
		if d[l]!=k-1:
			d[l]+=1
			cur+=1
		else:
			d[l]=0
			cur-=k-1
		# print(d)
		flag=0
		for kk in range(ord(s[j])+1,123):
			if d[kk]>0:
				if cur-1<n-j:
					cur-=1
					s[j]=chr(kk)
					d[kk]-=1
					d[97]+=n-j-1-cur
					ans=s[:j+1]
					for jj in sorted(d):
						ans+=[chr(jj)]*d[jj]
					# print(s)
					flag=1
					break
			else:
				if cur+k-1<n-j:
					cur+=k-1
					s[j]=chr(kk)
					d[kk]+=k-1
					d[97]+=n-j-1-cur
					ans=s[:j+1]
					for jj in sorted(d):
						ans+=[chr(jj)]*d[jj]
					# print(s)
					flag=1
					break
		if flag:
			break

	print(''.join(ans))