a, b = input().split()

if len(a) > len(b):
    print(a)
elif len(a) < len(b):
    print(b)
else:
    for i in range(len(a)):
        if a[i] != b[i]:
            if a[i] == "4" and b[i] == "7":
                print(a)
                break
            elif a[i] == "7" and b[i] == "4":
                print(b)
                break
            else:
                a = int(a)
                b = int(b)
                if a > b:
                    print(a)
                    break
                else:
                    print(b)
                    break