def read_data():
    N = int(input())
    L = int(input())
    Xs = [int(input()) for i in range(N - 1)]
    return N, L, Xs

def solve(N, L, Xs):
    Xs.reverse()
    Xs.append(0)
    cum = 0
    n = 0
    centers = []
    for x in Xs:
        cum += x
        n += 1
        center = (cum / n) + L/2
        centers.append(center)
    bonds = 0
    for i in range(N-1):
        center = centers[i]
        x_top = Xs[i]
        x_bottom = Xs[i + 1]
        left = max(x_top, x_bottom)
        right = min(x_top + L, x_bottom + L)
        if left >= right:
            return -1
        if left < center < right:
            pass
        else:
            bonds += 1
    return bonds

N, L, Xs = read_data()
print(solve(N, L, Xs))