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

#------------------------template--------------------------#
import os
import sys
from math import *
from collections import *
# from fractions import *
from heapq import*
from bisect import *
from io import BytesIO, IOBase
def vsInput():
    sys.stdin = open('input.txt', 'r')
    sys.stdout = open('output.txt', 'w')
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")
ALPHA='abcdefghijklmnopqrstuvwxyz'
M=998244353
EPS=1e-6
def Ceil(a,b): return a//b+int(a%b>0)
def value():return tuple(map(int,input().split()))
def array():return [int(i) for i in input().split()]
def Int():return int(input())
def Str():return input()
def arrayS():return [i for i in input().split()]


#-------------------------code---------------------------#
# vsInput()




for _ in range(Int()):

    n,k = value()
    s = input()
    s = s[::-1]

    C = Counter(s)
    ans = s[::-1]

    for i in C:
        if(C[i]%k): ans = -1
    
    for i in range(n):
        
        C[s[i]] -= 1
        if(s[i] == 'z' or ans != -1): continue
        next = False

        need = 0
        have = i + 1
        for ch in ALPHA[::-1]:
            need += (k - C[ch]%k )%k
            if((k - C[ch]%k )%k and ord(ch) == 1+ord(s[i])): next = ch

        # print(i,need,have,next,C)
        if(need > have or (have - need)%k): continue
        if(have == need and not next): continue

        ans = s[i+1:][::-1]
        needed = []
        for ch in ALPHA: needed += [ch]*((k - C[ch]%k )%k)

        if(not next): 
            next = ALPHA[ ALPHA.index(s[i]) + 1 ]
            needed += [next]*(k)
            have -= k
            needed.sort()
            
        # print(needed,i,next,ans)
        needed.remove(next)
        needed.insert(0,next)
        needed += ['a']*(have - need)
        ans += ''.join(needed)
        break

    print(ans)
            
        
        




















