A,B = map(str,input().split())
if len(A) > len(B): print(A)
elif len(A) < len(B): print(B)
else:
    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