N = input()
m = int(N[-1])
N = int(N[0])
blocks = list(map(int, input().split(" ")))

if min(blocks) + max(blocks) > N:
    print(0)

elif sum(blocks *2) < N:
    print(-1)

else:
    res = []
    for x in blocks:
        if N-x in blocks:
            res.append(x)
            res.append(N-x)
        elif N - x*2 in blocks:
            res.append(x)
            res.append(x)
            res.append(N-2*x)
    print(len(res))
    print(" ".join(map(str, res)))