# -*- coding: utf-8 -*-

import sys
import math

n = int(sys.stdin.readline())
l = int(sys.stdin.readline())
hl = float(l) / 2
args = sys.stdin.readlines()
cntr = 0

def grav_sum(i, n, prev_sum, xi):
    return float((n-i-1)*prev_sum + xi)/(n-i)

if n == 1:
    pass
else:
    xi = float(args[-1])
    gsum = xi
    for i in reversed(range(len(args))):
        if i == 0:
            xii = xi
            xi = 0
        else:
            xii = xi
            xi = int(args[i-1])
        if math.fabs(xi - xii) > l - 1:
            cntr = -1
            break
        elif round(math.fabs(gsum - xi),10) < hl and round(math.fabs(gsum - xii),10) < hl:
            gsum = grav_sum(i, n, gsum, xi)
        else:
            gsum = grav_sum(i, n, gsum, xi)
            cntr += 1

print cntr