n, m = list(map(int, input().split()))
A = list(map(int, input().split()))
S = list(map(int, input().split()))
B = [0] * (n+1)
#C = [[0 for _ in range(n+1)] for _ in range(n+1)]
T = []
for s in S:
    B[s] = 1
    T.append(s)

while T:
    now = T.pop()
    for s in S:
        j = A[s + now]
        if not B[j]:
            B[j] = 1
            T.append(j)

print(sum(B))