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 re
import random
import math
import copy
from heapq import heappush, heappop, heapify
from functools import cmp_to_key
from bisect import bisect_left, bisect_right
from collections import defaultdict, deque, Counter
# sys.setrecursionlimit(1000000)

# input aliases
input = sys.stdin.readline
getS = lambda: input().strip()
getN = lambda: int(input())
getList = lambda: list(map(int, input().split()))
getZList = lambda: [int(x) - 1 for x in input().split()]

INF = float("inf")
MOD = 10**9 + 7
divide = lambda x: pow(x, MOD-2, MOD)


def nck(n, k, kaijyo):
    return (npk(n, k, kaijyo) * divide(kaijyo[k])) % MOD

def npk(n, k, kaijyo):
    if k == 0 or k == n:
        return n % MOD
    return (kaijyo[n] * divide(kaijyo[n-k])) % MOD

def fact_and_inv(SIZE):
    inv = [0] * SIZE  # inv[j] = j^{-1} mod MOD
    fac = [0] * SIZE  # fac[j] = j! mod MOD
    finv = [0] * SIZE  # finv[j] = (j!)^{-1} mod MOD
    inv[1] = 1
    fac[0] = fac[1] = 1
    finv[0] = finv[1] = 1
    for i in range(2, SIZE):
        inv[i] = MOD - (MOD // i) * inv[MOD % i] % MOD
        fac[i] = fac[i - 1] * i % MOD
        finv[i] = finv[i - 1] * inv[i] % MOD

    return fac, finv

def renritsu(A, Y):
    # example 2x + y = 3, x + 3y = 4
    # A = [[2,1], [1,3]])
    # Y = [[3],[4]] または [3,4]
    A = np.matrix(A)
    Y = np.matrix(Y)
    Y = np.reshape(Y, (-1, 1))
    X = np.linalg.solve(A, Y)

    # [1.0, 1.0]
    return X.flatten().tolist()[0]

class TwoDimGrid:
    # 2次元座標 -> 1次元
    def __init__(self, h, w, wall="#"):
        self.h = h
        self.w = w
        self.size = (h+2) * (w+2)
        self.wall = wall
        self.get_grid()
        # self.init_cost()

    def get_grid(self):
        grid = [self.wall * (self.w + 2)]
        for i in range(self.h):
            grid.append(self.wall + getS() + self.wall)

        grid.append(self.wall * (self.w + 2))
        self.grid = grid
    def init_cost(self):
        self.cost = [INF] * self.size

    def pos(self, x, y):
        # 壁も含めて0-indexed 元々の座標だけ考えると1-indexed
        return y * (self.w + 2) + x
    def getgrid(self, x, y):
        return self.grid[y][x]
    def get(self, x, y):
        return self.cost[self.pos(x, y)]
    def set(self, x, y, v):
        self.cost[self.pos(x, y)] = v
        return
    def show(self):
        for i in range(self.h+2):
            print(self.cost[(self.w + 2) * i:(self.w + 2) * (i+1)])
    def showsome(self, tgt):
        for t in tgt:
            print(t)
        return
    def showsomejoin(self, tgt):
        for t in tgt:
            print("".join(t))
        return

    def search(self):
        grid = self.grid
        move = [(0, 1), (0, -1), (1, 0), (-1, 0)]
        move_eight = [(0, 1), (0, -1), (1, 0), (-1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
        # for i in range(1, self.h+1):
        #     for j in range(1, self.w+1):
        #         cx, cy = j, i
        #         for dx, dy in move:
        #             nx, ny = dx + cx, dy + cy


class BIT():
    # A1 ... AnのBIT(1-indexed)
    def __init__(self, n):
        self.n = n
        self.BIT = [0] * (n + 2)

    # A1 ~ Aiまでの和 O(logN)
    def query(self, idx):
        res_sum = 0
        while idx > 0:
            res_sum += self.BIT[idx]
            idx -= idx & (-idx)
        return res_sum

    # Ai += x O(logN)
    def update(self, idx, x):
        # print(idx)
        while idx <= self.n:
            # print(idx)
            self.BIT[idx] += x
            idx += idx & (-idx)
        return

    def show(self):
        print(self.BIT)

def judge(cnt, c, rem, zan):
    for i in range(ord(c)+1, ord("z") + 1):
        moji = chr(i)
        cnt[moji] += 1
        app = []
        for k, v in cnt.items():
            if v % rem == 0:
                continue
            for j in range(rem - (v % rem)):
                app.append(k)
                zan -= 1
        if zan < 0 or zan % rem != 0:
            cnt[moji] -= 1
            continue

        app.sort()
        return "".join([moji] + app)



    return []


def first(cnt, m):
    for k, v in cnt.items():
        if v % m != 0:
            return False

    return True
def solve():
    n, m = getList()
    s = input().strip()
    cnt = Counter(s)
    # print(cnt)

    if first(cnt, m):
        print(s)
        return

    zan = 0
    for c in list(s)[::-1]:
        cnt[c] -= 1
        kuttuki = judge(cnt, c, m, zan)
        if kuttuki:
            # print(kuttuki)
            print(s[:-(1+zan)] + kuttuki)
            return
        zan += 1

    print(-1)

def main():
    n = getN()
    for _ in range(n):
        solve()

    return
if __name__ == "__main__":
    main()
    # solve()