Given an n-sided regular polygon n, return the total sum of internal angles (in degrees).

Examples
sumPolygon(3) ➞ 180
sumPolygon(4) ➞ 360
sumPolygon(6) ➞ 720
Notes
n will always be greater than 2.
The formula (n - 2) x 180 gives the sum of all the measures of the angles of an n-sided polygon.