Rectangular wall is 4m wide n2.5m high1litre paint is needed to cover the wall.find thicknes of coat of paint

pls help i don't understand what formula to use. the answer should be in millilitre.thanks

1 Liter is = 1,000 cubic centimetres (cm3)
= 1,000 * (10*10*10) (mm3)

Area = 4m*2.5m
=4*100cm*2.5*100cm
=4*100*10mm*2.5*100*10mm

Thickness = 1,000 * (10*10*10) (mm3) / 4*100*10mm*2.5*100*10mm
=0.1 mm