n, m = list(map(int, input().split()))
a = list(map(int, input().split()))
b = list(map(int, input().split()))

def check ():
    res = 1
    for i in range(n):
        for j in range(m):
            if a[i] == b[j]:
                print(res)
                return
            res += 1

    print(-1)

check()