import sys
import math


def is_invalid(L: int, xs: list[int]) -> bool:
    for i in range(len(xs) - 1):
        if abs(xs[i] - xs[i + 1]) >= L:
            return True
    return False


def calc(N: int, L: int, xs: list[int]) -> int:
    if is_invalid(L, xs):
        return -1

    ans = 0
    p = (xs[-1] + xs[-1] + L) / 2.0
    cnt = 1

    for i in range(len(xs) - 2, -1, -1):
        t = p / cnt
        if (xs[i] < t < xs[i] + L) and (xs[i + 1] < t < xs[i + 1] + L):
            pass
        else:
            ans += 1

        p += (xs[i] + xs[i] + L) / 2.0
        cnt += 1

    return ans


N = int(input())
L = int(input())
xs = [0]
for _ in range(N-1):
    xs.append(int(input()))

ans = calc(N, L, xs)
print(ans)