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
read=sys.stdin.buffer.read;readline=sys.stdin.buffer.readline;input=lambda:sys.stdin.readline().rstrip()
import bisect,string,math,time,functools,random,fractions
from bisect import*
from heapq import heappush,heappop,heapify
from collections import deque,defaultdict,Counter
from itertools import permutations,combinations,groupby
rep=range;R=range
def I():return int(input())
def LI():return [int(i) for i in input().split()]
def LI_():return [int(i)-1 for i in input().split()]
def AI():return map(int,open(0).read().split())
def S_():return input()
def IS():return input().split()
def LS():return [i for i in input().split()]
def NI(n):return [int(input()) for i in range(n)]
def NI_(n):return [int(input())-1 for i in range(n)]
def NLI(n):return [[int(i) for i in input().split()] for i in range(n)]
def NLI_(n):return [[int(i)-1 for i in input().split()] for i in range(n)]
def StoLI():return [ord(i)-97 for i in input()]
def ItoS(n):return chr(n+97)
def LtoS(ls):return ''.join([chr(i+97) for i in ls])
def RLI(n=8,a=1,b=10):return [random.randint(a,b)for i in range(n)]
def RI(a=1,b=10):return random.randint(a,b)
def INP():
    N=100
    x=random.randint(1,N)
    y=random.randint(1,N)
    p=random.randint(1,N)
    q=random.randint(1,N)
    return x,y,p,q
def Rtest(T):
    case,err=0,0
    for i in range(T):
        inp=INP()
        a1=naive(*inp)
        a2=solve(*inp)
        if a1!=a2:
            #print(inp[0],*inp[1])
            x,y,p,q=inp
            c=2*(x+y)
            r=p+q
            print(inp,(c,r),gcd(c,r),((x,x+y),(p,p+q)))
            print('naive',a1)
            print('solve',a2)
            err+=1
        case+=1
    print('Tested',case,'case with',err,'errors')
def GI(V,E,ls=None,Directed=False,index=1):
    org_inp=[];g=[[] for i in range(V)]
    FromStdin=True if ls==None else False
    for i in range(E):
        if FromStdin:
            inp=LI()
            org_inp.append(inp)
        else:
            inp=ls[i]
        if len(inp)==2:a,b=inp;c=1
        else:a,b,c=inp
        if index==1:a-=1;b-=1
        aa=(a,c);bb=(b,c);g[a].append(bb)
        if not Directed:g[b].append(aa)
    return g,org_inp
def GGI(h,w,search=None,replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1):
    #h,w,g,sg=GGI(h,w,search=['S','G'],replacement_of_found='.',mp_def={'#':1,'.':0},boundary=1) # sample usage
    mp=[boundary]*(w+2);found={}
    for i in R(h):
        s=input()
        for char in search:
            if char in s:
                found[char]=((i+1)*(w+2)+s.index(char)+1)
                mp_def[char]=mp_def[replacement_of_found]
        mp+=[boundary]+[mp_def[j] for j in s]+[boundary]
    mp+=[boundary]*(w+2)
    return h+2,w+2,mp,found
def TI(n):return GI(n,n-1)
def accum(ls):
    rt=[0]
    for i in ls:rt+=[rt[-1]+i]
    return rt
def bit_combination(n,base=2):
    rt=[]
    for tb in R(base**n):s=[tb//(base**bt)%base for bt in R(n)];rt+=[s]
    return rt
def gcd(x,y):
    if y==0:return x
    if x%y==0:return y
    while x%y!=0:x,y=y,x%y
    return y
def YN(x):print(['NO','YES'][x])
def Yn(x):print(['No','Yes'][x])
def show(*inp,end='\n'):
    if show_flg:print(*inp,end=end)

mo=10**9+7
#mo=998244353
inf=float('inf')
FourNb=[(-1,0),(1,0),(0,1),(0,-1)];EightNb=[(-1,0),(1,0),(0,1),(0,-1),(1,1),(-1,-1),(1,-1),(-1,1)];compas=dict(zip('WENS',FourNb));cursol=dict(zip('LRUD',FourNb))
alp=[chr(ord('a')+i)for i in range(26)]
#sys.setrecursionlimit(10**7)

show_flg=False
#show_flg=True


ans=inf

def solve(s,mx,n,k):
    d=Counter(s[:mx])
    rt=[]
    dc=defaultdict(int)
    for i,c in d.items():
        rt+=[i]*( (c+k-1)//k*k-c )
        dc[i]+=( (c+k-1)//k*k-c )
    
    ans=[]
    show('sorted(rt)[::-1]<s[mx:]',sorted(rt)[::-1],s[mx:])
    if sorted(rt)[::-1]<s[mx:]: # maeno sorted(rt)[::-1][-1] made modoru
        minc=sorted(rt)[0]
        for i in range(mx-1,-1,-1):
            if s[i]==minc:
                st=i
                break
            dc[s[i]]+=1
        dc[minc]+=1
        ans=s[:st]+[minc+1]
        dc[minc+1]+=-1
        rm=[]
        if minc+1==26:
            return -1
        for p,q in dc.items():
            if q!=0:
                rm+=[p]*q
        ans+=sorted(rm)
    else: # ushiro wo mite narabeteku
        ans=s[:mx]
        for j in range(mx,n):
            if dc[s[j]]>0:
                dc[s[j]]-=1
                ans+=s[j],
            else:
                rm=[]
                for p,q in dc.items():
                    if q!=0:
                        rm+=[p]*q
                ans+=sorted(rm)
    return LtoS(ans)


def remain(s,mx,n,k):
    d=Counter(s[:mx])
    rt=[]
    for i,c in d.items():
        rt+=[i]*( (c+k-1)//k*k-c )
    show(d)
    if sorted(rt)<s[mx:]:
        return 'impossible',LtoS(rt)
        
    return LtoS(rt)

for _ in range(I()):
    an=None
    n,k=LI()
    s=StoLI()
    ss=LtoS(s)
    
    if n%k!=0:
        ans=-1
        print(ans)
        continue
    
    d=defaultdict(int)
    
    t=0
    for i in range(n):
        c=s[i]
        d[c]+=1
        if k==1 or d[c]%k==1:
            t+=k
        if t==n+k:
            an=solve(s,i,n,k)
            #ans=remain(s,i,n,k)
            show('!',ss,i,s[:i],ans,Counter(s[:i]),'ans',an)
            
    #print(an,'!',_)
        #show(ss,chr(c+97),t)
    
    
    if an==None:
        print(ss)
    else:
        print(an)
        

    



