A, B = input().split()

if len(A) != len(B):
    print(A if len(A) > len(B) else B)
    exit()

ans = None
for i in range(len(A) - 1, -1, -1):
    a, b = A[i], B[i]
    if a == b:
        continue
    elif a == '4' and b == '7':
        ans = A
    elif a == '7' and b == '4':
        ans = B
    else:
        ans = (A if a > b else B)
print(ans)