func main()
	var n: int :: cui@inputInt()
	var k: int :: cui@inputInt()
	var a: []int :: #[n]int
	for i(0, n - 1)
		do a[i] :: cui@inputInt()
	end for
	var cnt: int :: 0
	for start(0, k - 1)
		var arr: []int :: #[0]int
		var idx: int :: start
		while(idx < n)
			for j(0, ^arr - 1)
				if(a[idx] < arr[j])
					do cnt :+ 1
				end if
			end for
			do arr :~ [a[idx]]
			do idx :+ k
		end while
		do arr.sort()
		for i(0, ^arr - 1)
			do a[start + i * k] :: arr[i]
		end for
	end for
	for i(1, n - 1)
		if(a[i] < a[i - 1])
			do cui@print("-1\n")
			ret
		end if
	end for
	var ans: int :: cnt
	do cui@print("\{ans}\n")
end func