A,B = map(str,input().split())
if len(A) > len(B): print(A)
elif len(A) < len(B): print(B)
i = 0
while i < len(A):
    if A[i] == '4':
        if B[i] == '7':
            print(A)
            break
    if A[i] == '7':
        if B[i] == '4':
            print(B)
            break
    if A[i] > B[i]:
        print(A)
        break
    elif A[i] < B[i]: 
        print(B)
        break
    i = i+1