```go
package main

import (
	"bufio"
	"fmt"
	"os"
)

func main() {
	reader := bufio.NewReader(os.Stdin)
	writer := bufio.NewWriter(os.Stdout)
	defer writer.Flush()

	var T int
	fmt.Fscan(reader, &T)
	for t := 0; t < T; t++ {
		var n, k int
		var s string
		fmt.Fscan(reader, &n, &k)
		fmt.Fscan(reader, &s)

		if n%k != 0 {
			fmt.Fprintln(writer, -1)
			continue
		}

		cnt := [26]int{}
		for i := 0; i < n; i++ {
			cnt[s[i]-'a']++
		}

		need := 0
		for i := 0; i < 26; i++ {
			need += (k - cnt[i]%k) % k
		}

		if need == 0 {
			fmt.Fprintln(writer, s)
			continue
		}

		found := false
		ans := make([]byte, n)

		for i := n - 1; i >= 0; i-- {
			cidx := s[i] - 'a'

			old := (k - cnt[cidx]%k) % k
			cnt[cidx]--
			new := (k - cnt[cidx]%k) % k
			need += new - old

			for nc := cidx + 1; nc < 26; nc++ {
				old = (k - cnt[nc]%k) % k
				cnt[nc]++
				new = (k - cnt[nc]%k) % k
				need += new - old

				if need <= n-i-1 {
					copy(ans, s[:i])
					ans[i] = byte('a' + nc)

					for pos := i + 1; pos < n; pos++ {
						for d := 0; d < 26; d++ {
							old_d := (k - cnt[d]%k) % k
							cnt[d]++
							new_d := (k - cnt[d]%k) % k
							need += new_d - old_d

							if need <= n-pos-1 {
								ans[pos] = byte('a' + d)
								break
							}

							cnt[d]--
							need -= new_d - old_d
						}
					}

					fmt.Fprintln(writer, string(ans))
					found = true
					break
				}

				cnt[nc]--
				need -= new - old
			}

			if found {
				break
			}
		}

		if !found {
			fmt.Fprintln(writer, -1)
		}
	}
}
```