package main

import (
	"bufio"
	"bytes"
	"io"
	"os"
	"strconv"
)

func main() {
	data, _ := io.ReadAll(os.Stdin)
	idx := 0

	nextInt := func() int {
		for idx < len(data) && data[idx] <= ' ' {
			idx++
		}
		sign := 1
		if idx < len(data) && data[idx] == '-' {
			sign = -1
			idx++
		}
		val := 0
		for idx < len(data) && data[idx] >= '0' && data[idx] <= '9' {
			val = val*10 + int(data[idx]-'0')
			idx++
		}
		return sign * val
	}

	t := nextInt()
	var out bytes.Buffer
	const inf = int(1e9)

	for ; t > 0; t-- {
		n := nextInt()
		k := nextInt()
		a := make([]int, n+1)
		for i := 1; i <= n; i++ {
			a[i] = nextInt()
		}

		dp := make([]int, n+1)
		ans := inf

		for i := 1; i <= n; i++ {
			if a[i] > i {
				continue
			}
			dp[i] = 1
			ci := i - a[i]
			for j := 1; j < i; j++ {
				if dp[j] > 0 && a[j] < a[i] && j-a[j] <= ci {
					if dp[j]+1 > dp[i] {
						dp[i] = dp[j] + 1
					}
				}
			}
			if dp[i] >= k && ci < ans {
				ans = ci
			}
		}

		if ans == inf {
			out.WriteString("-1\n")
		} else {
			out.WriteString(strconv.Itoa(ans))
			out.WriteByte('\n')
		}
	}

	w := bufio.NewWriter(os.Stdout)
	w.Write(out.Bytes())
	w.Flush()
}