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

check = [False for i in range(M+1)]
done = [False for i in range(M+1)]

for i in B:
    check[i] = True
    done[i] = True
check[0] = False
done[0] = False
state = True

out = []
for i in range(1,N):
    i = B[i]
    if done[i]:
        out.append(i)
        for j in range(i,M+1,i):
            if not check[j]:
                state = False
            done[j] = False

if state:
    print(len(out))
    [print(i, end=" ") for i in out]
else:print(-1)