A, B = raw_input().split()

if len(A) == len(B):
    for i in xrange(len(A)):
        if A[i] == B[i]:
            continue
        else:
            if int(A) > int(B):
                if A[i] != '7' or B[i] != '4':
                    print A
                    break
                else:
                    print B
                    break
            else:
                if A[i] != '4' or B[i] != '7':
                    print B
                    break
                else:
                    print A
                    break

else:
    print A if len(A) > len(B) else B