J=lambda:map(int,input().split())
N,M=J()
A=list(J())
b=[0]*(N+1)
S=[]
for s in J():
	b[s]=1
	S+=[s]
T=S[:]
while(len(T)):
	j=T.pop(-1)
	for i in S:
		t=A[i+j]
		if b[t]<1:
			b[t]=1
			T+=[t]
print(sum(b))