N,M=map(int,input().split())
A=list(map(int,input().split()))
B=list(map(int,input().split()))
d=[0]*(N+1)
from collections import deque
S=deque()
for x in B:
  S.append(x)
  d[x]=1
while S:
  pos=S.pop()
  for x in B:
    pos2=pos+x
    y=A[pos2]
    if d[y]==0:
      d[y]=1
      S.append(y)
print(sum(d))