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

N = int(input())
L = int(input())

if N == 1:
    print(0)
    exit()

X = [None] * N
X[0] = 0
for i in range(1, N):
    X[i] = int(input())

numer = 0
denom = 0
ans = N-1
for i in range(N-2, 0-1, -1):
    if abs(X[i+1] - X[i]) >= L:
        print(-1)
        exit()

    numer += X[i+1]
    denom += 1
    if X[i] < numer/denom + L/2 < X[i] + L and X[i+1] < numer/denom + L/2 < X[i+1] + L:
        ans -= 1

print(ans)