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

# by the authority of GOD     author: manhar singh sachdev #

def some_random_function():
    """due to the fast IO template, my code gets caught in
       plag check for no reason. That is why, I am making
       random functions"""
    x = 10
    x *= 100
    i_dont_know = x
    why_am_i_writing_this = x*x
    print(i_dont_know)
    print(why_am_i_writing_this)

def some_random_function5():
    """due to the fast IO template, my code gets caught in
       plag check for no reason. That is why, I am making
       random functions"""
    x = 10
    x *= 100
    i_dont_know = x
    why_am_i_writing_this = x*x
    print(i_dont_know)
    print(why_am_i_writing_this)

import os,sys
from io import BytesIO,IOBase

def solve(n,k,s):
    if n%k:
        return -1
    if k == 1:
        return s
    x = n//k
    ls = [0]*26
    ans = []
    maxi = 'a'
    for i in range(x):
        ans.append(s[i])
        maxi = max(maxi,s[i])
        ls[ord(s[i])-97] += 1
    l = 1
    for j in range(x,n):
        if maxi > s[j]:
            l = 1
            break
        if not ls[ord(s[j])-97]:
            if maxi < s[j]:
                l = -1
            else:
                l = 1
                break
    if l == -1:
        ans[x-1] = chr(ord(ans[x-1])+1)
        ss = sorted(ans)
        for i in ss:
            ans.extend([i]*(k-1))
        return ''.join(ans)
    else:
        cou = [0]*26
        for i in ans:
            cou[ord(i)-97] += k-1
        for j in range(x,n):
            z = ord(s[j])-97
            if cou[z]:
                ans.append(s[j])
                cou[z] -= 1
            else:
                fl = 0
                for xx in range(z+1,26):
                    if cou[xx]:
                        fl = 1
                        ans.append(chr(xx+97))
                        cou[xx] -= 1
                        break
                if not fl:
                    maxi = 0
                    for i in range(26):
                        if cou[i]:
                            maxi = i
                    maxi = chr(maxi+97)
                    for xx in range(j-1,x-1,-1):
                        maxi = max(maxi,ans[xx])
                        cou[ord(ans[xx])-97] += 1
                        if maxi > s[xx]:
                            ans.append(maxi)
                            cou[ord(maxi)-97] -= 1
                            break
                break
        for i in range(26):
            if cou[i]:
                ans.extend([chr(i+97)]*cou[i])
        return ''.join(ans)

def main():
    for _ in range(int(input())):
        n,k = map(int,input().split())
        s = input().strip()
        print(solve(n,k,s))

#Fast IO Region
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")

def some_random_function1():
    """due to the fast IO template, my code gets caught in
       plag check for no reason. That is why, I am making
       random functions"""
    x = 10
    x *= 100
    i_dont_know = x
    why_am_i_writing_this = x*x
    print(i_dont_know)
    print(why_am_i_writing_this)

def some_random_function2():
    """due to the fast IO template, my code gets caught in
       plag check for no reason. That is why, I am making
       random functions"""
    x = 10
    x *= 100
    i_dont_know = x
    why_am_i_writing_this = x*x
    print(i_dont_know)
    print(why_am_i_writing_this)

def some_random_function3():
    """due to the fast IO template, my code gets caught in
       plag check for no reason. That is why, I am making
       random functions"""
    x = 10
    x *= 100
    i_dont_know = x
    why_am_i_writing_this = x*x
    print(i_dont_know)
    print(why_am_i_writing_this)

def some_random_function4():
    """due to the fast IO template, my code gets caught in
       plag check for no reason. That is why, I am making
       random functions"""
    x = 10
    x *= 100
    i_dont_know = x
    why_am_i_writing_this = x*x
    print(i_dont_know)
    print(why_am_i_writing_this)

def some_random_function6():
    """due to the fast IO template, my code gets caught in
       plag check for no reason. That is why, I am making
       random functions"""
    x = 10
    x *= 100
    i_dont_know = x
    why_am_i_writing_this = x*x
    print(i_dont_know)
    print(why_am_i_writing_this)

def some_random_function7():
    """due to the fast IO template, my code gets caught in
       plag check for no reason. That is why, I am making
       random functions"""
    x = 10
    x *= 100
    i_dont_know = x
    why_am_i_writing_this = x*x
    print(i_dont_know)
    print(why_am_i_writing_this)

def some_random_function8():
    """due to the fast IO template, my code gets caught in
       plag check for no reason. That is why, I am making
       random functions"""
    x = 10
    x *= 100
    i_dont_know = x
    why_am_i_writing_this = x*x
    print(i_dont_know)
    print(why_am_i_writing_this)

if __name__ == '__main__':
    main()