n,m=map(int,input().split())
a=list(map(int,input().split()))
b=list(map(int,input().split()))
x=[]
y=[]
for i in range(m):
    for nod in a:x.append(nod)

for i in range(n):
    for nod in b:y.append(nod)

for i in range(n*m):
    if x[i]==y[i]:
        print(i+1)
        exit()
print(-1)