N,M = map(int,input().split())
A = list(map(int,input().split()))
B = list(map(int,input().split()))
flg = False
p = []
ans = float('inf')
for i in range(N):
  for j in range(M):
    if A[i] == B[j]:
      flg = True
      num = 0
      while num*N+i+1 < N*M+1:
        m = (num*N+i-j)/M
        if m.is_integer():
          break
        num += 1
      ans = min(ans,num*N+i+1)

if flg:
  print(ans)
else:
  print(-1)