def main():
    A, B = input().split()
    a, b = [x.zfill(10) for x in (A, B)]
    ans = -1
    for i in range(10):
        if a[i] == b[i]: continue
        if a[i] == '4' and b[i] == '7':
            ans = A
        elif a[i] == '7' and b[i] == '4':
            ans = B
        elif a[i] > b[i]:
            ans = A
        else:
            ans = B
        break
    print(ans)

main()