from itertools import accumulate
import sys
input = sys.stdin.buffer.readline
sys.setrecursionlimit(10 ** 7)

N = int(input())
L = int(input()) * 2
X = [0] + [int(input()) * 2 for _ in range(N-1)]

# 破綻してないかチェック
if any(abs(a - b) >= L for a, b in zip(X[:-1], X[1:])):
    print(-1)
    exit()


def safe(i, n, g):
    return n * left[i] < g and g < n * right[i]


right = [c + L // 2 for c in X]
left = [c - L // 2 for c in X]

ans = 0
g = 0  # 重心
n = 0  # 上にあるパネル数
for i in reversed(range(1, N)):
    n += 1
    g += X[i]
    if safe(i, n, g) and safe(i - 1, n, g):
        continue
    ans += 1

print(ans)