n, m = map(int, input().split())
A = list(map(int, input().split()))
B = list(map(int, input().split()))

i = j = 0
x = 1
ans = -1
for _ in range(n * m):
    if A[i] == B[j]:
        ans = x
        break
    x += 1
    if i == n - 1:
        i = 0
    else:
        i += 1
    if j == m - 1:
        j = 0
    else:
        j += 1
    
print(ans)