For a convex 18-gon what is the sum of the interior angles ?

I'm terrible at math any here is appreciated.

Extra points! find the missing dimension! need asap

You are standing next to a really big circular lake. you want to measure the diameter of the lake, but you don't want to have to swim across with a measuring tape! you decide to walk around the perimeter of the lake and measure its circumference, and find that it's 400\pi\text{ m}400? m. what is the diameter dd of the lake?