A0,B0 = input().split()
A,B = list(A0),list(B0)
for i in range(len(A)):
    if A[i] == "4" and B[i] == "7":
        A[i],B[i] = "7","4"
    elif A[i] == "7" and B[i] =="4":
        A[i],B[i] = "4","7"
A1 = int("".join(A))
B1 = int("".join(B))
if A1 > B1:
    print(int(A0))
else:
    print(int(B0))