a, b = gets.split
if a.size > b.size
    puts a
elsif a.size < b.size
    puts b
else
    n = "-1"
    a.size.times do |i|
        next if a[i] == b[i]
        if [a[i], b[i]].sort == ["4", "7"]
            n = a[i] == "4" ? a : b
            break
        else
            n = a[i] > b[i] ? a : b
            break
        end
    end
    n = a if n == "-1"
    puts n
end