import sys
N, M = map(int, sys.stdin.readline().split())
A = list(map(int, sys.stdin.readline().split()))
B = list(map(int, sys.stdin.readline().split()))



if(len(A) <= len(B)):
    bx = (M - N) * N
    ax = (bx // N) + 2
else:
    ax = (N - M) * M
    bx = (ax // M) + 2
b = B * bx
a = A * ax

#print(a)
#print(b)
fois = min(len(a), len(b))
#print(ax, bx, fois)

for i in range(0, fois):
    if a[i] == b[i]:
        print(i+1)
        exit()
print(-1)