def prt(x):
    print(x)
    exit()

A,B = map(str,input().split())
if len(A) > len(B):
    prt(A)
if len(A) < len(B):
    prt(B)
for i in range(len(A)):
    if A[i] == "7" and B[i] == "4":
        prt(B)
    if A[i] == "4" and B[i] == "7":
        prt(A)
    if A[i] > B[i]:
        prt(A)
    if A[i] < B[i]:
        prt(B)