import numpy as np
N, M = map(int, input().split())
A = list(map(int, input().split()))
B = list(map(int, input().split()))

arrayA =np.array(list(A)*10000)
arrayB =np.array(list(B)*10000)

cnt=0
i=1
while i <=10000:
    if arrayA[i-1] == arrayB[i-1]:
        print(i)
        cnt=1
        break
    else:
        i += 1

if cnt == 1:
    print()
else:
    print(-1)