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

import sys
import math

args = sys.stdin.readlines()
n = float(args[0])
hl = float(args[1]) / 2

def loop(i, prev_sum, n, cntr):
    xii = float(args[i+1])
    if i == 1:
        if math.fabs(float(args[i+1])) > 2*hl-1:
            return -1
        elif math.fabs(prev_sum) < hl and math.fabs(prev_sum - xii) < hl:
            return cntr
        else:
            return cntr+1
    else:
        xi = float(args[i])
        if math.fabs(xi - xii) > 2*hl-1:
            return -1
        elif math.fabs(prev_sum - xi) < hl and math.fabs(prev_sum - xii) < hl:
            new_sum = float((n-i)*prev_sum + xi)/(n-i+1)
            return loop(i-1, new_sum, n, cntr)
        else:
            new_sum = float((n-i)*prev_sum + xi)/(n-i+1)
            return loop(i-1, new_sum, n, cntr+1)


print loop( len(args)-2 , float(args[len(args)-1]), n, 0 )