import numpy as np

N, K = map(int, input().split())
A = tuple(map(int, input().split()))

dp1 = np.zeros((N + 1, K + 1), dtype=bool)
dp2 = np.zeros((N + 1, K + 1), dtype=bool)

dp1[:, 0] = True
dp2[:, 0] = True

for i in range(N):
    a = A[i]
    dp1[i + 1] = dp1[i]
    dp1[i + 1, a:] |= dp1[i, :K + 1 - a]
for i in range(N - 1, -1, -1):
    a = A[i]
    dp2[i] = dp2[i + 1]
    dp2[i, a:] |= dp2[i + 1, :K + 1 - a]

if not dp1[-1, K]:
    print(-1)
    exit()

ans = np.count_nonzero(~np.any(dp1[:-1] & dp2[1:, ::-1], axis=1))
print(ans)