def main():
    N, M = map(int, input().split())
    A = list(map(int,(input().split()))) * M
    B = list(map(int,(input().split()))) * N
    
    for i in range(N * M):
        if A[i] == B[i]:
            print(i + 1)
            return
    else:
        print(-1)

if __name__ == '__main__':
    main()