N,M = map(int,input().split())
A = list(map(int,input().split()))
B = list(map(int,input().split()))

A2 = A*1000
B2 = B*1000
ans = 10**18
for i in range(min(N*1000,M*1000)):
    if A2[i] == B2[i]:
        ans = i+1
        break
if ans == 10**18:
    print(-1)
else:
    print(ans)