\[\boxed{\text{266\ (266).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[999 + 1 = 1000\]
\[1000 - 1 = 999\]
\[10000 - 1 = 9\ 999\]
\[9\ 999 + 1 = 10\ 000\]
\[50\ 000 - 1 = 49\ 999\]
\[80\ 000 - 1 = 79\ 999\]