A river has a flow of $1000$ million litres per day (MLD), $BOD_5$ of $5\:mg/litre$ and Dissolved Oxygen (DO) level of $8\:mg/litre$ before receiving the wastewater discharge at a location. For the existing environmental conditions, the saturation DO level is $10\:mg/litre$ in the river. Wastewater discharge of $100$ MLD with the $BOD_5$ of $200\:mg/litre$ and DO level of $2\:mg/litre$ falls at that location. Assuming complete mixing of wastewater and river water, the immediate DO deficit (in $mg/litre$ round off to two decimal places), is ______.