Ok not a self sufficiency question, but there must be some mathematicians here. Normally I'm good at maths but for some reason this one has me stumped.

GCSE practice paper.

Sum of angles in a polygon of n sides = 180n-360.

A polygon has 2 corners with internal angle 90 degrees. The remaining corners all have angles (internal presumably) of 144 degrees. How many sides does the polygon have?