A, B = input().split()

def comp(A, B):
    if len(A) > len(B):
        return A
    elif len(A) < len(B):
        return B
    for a, b in zip(A, B):
        if a == '4' and b == '7':
            return A
        elif a == '7' and b == '4':
            return B
        elif a > b:
            return A
        elif a < b:
            return B

print(comp(A, B))