package main

import (
	"fmt"
)

func main() {
	var n, k int
	if _, err := fmt.Scan(&n, &k); err != nil {
		return
	}

	x := make([]int, n)
	for i := 0; i < n; i++ {
		fmt.Scan(&x[i])
	}

	ans := 0
	curr := 0
	for curr < n-1 {
		nextCurr := curr
		for nextCurr+1 < n && x[nextCurr+1]-x[curr] <= k {
			nextCurr++
		}
		if nextCurr == curr {
			fmt.Println("-1")
			return
		}
		curr = nextCurr
		ans++
	}
	fmt.Println(ans)
}