https://blog.ef67daisuki.club/2016/04/cpp%e3%81%a7%e3%83%90%e3%82%a4%e3%83%8a%e3%83%aa%e3%83%95%e3%82%a1%e3%82%a4%e3%83%ab%e3%82%92unsigned-char%e3%81%a7%e8%aa%ad%e3%81%bf%e8%be%bc%e3%82%80/