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

for i in range(N - 1):
    if abs(X[i + 1] - X[i]) >= L:
        print(-1)
        exit()

S = 0
ans = N - 1
for i in range(1, N):
    S += X[N - i]
    M = S / i + L / 2
    if X[N - i - 1] < M < X[N - i - 1] + L and X[N - i] < M < X[N - i] + L:
        ans -= 1
print(ans)