A,B=gets.split
As=A.size;Bs=B.size
if(As!=Bs) then puts [A.to_i, B.to_i].max; exit; end
(0...As).each{|i|
    if A[i]!=B[i]
        # puts "A[i]=#{A[i]} B[i]=#{B[i]}"
        if (A[i] > B[i] || (A[i]=='4' && B[i]=='7')) && !(A[i]=='7' && B[i]=='4')
            puts A; exit
        else
            puts B; exit
        end
    end
}