A, B = input().split()

ans = A
Al, Bl = len(A), len(B)

if Al < Bl:
    ans = B

if Al == Bl:
    for i in range(Al):
        if A[i] == '7' and B[i] == '4':
            ans = B
            break
        elif (A[i] != '4' or B[i] != '7') and A[i] < B[i]:
            ans = B
            break

print(ans)