Turtle Polygon drawing program

[tp_oz]

Hello everyone, my name's Tim and I'm a teacher and guess what: I've run out of coding skills

I'm going through the thinkcspy learning program to get a good grip on the foundations of python before I do really funky stuff with networks and robots and stuff.

I'm writing a polygon drawing program with the turtle module. Here's the code:

sides_choice = input("how many sides?")
sides = int(sides_choice)
print (sides)
length_choice = input("how long are the sides?")
length = int(length_choice)
print (length)
lines = input("what colour lines?")
print (lines)
fill = input("what colour fill?")
print (fill)

import turtle
wn = turtle.Screen()
bob = turtle.Turtle()

bob.color(lines)
bob.fillcolor(fill)
for polygon in range(sides):
    bob.fd(length)
    bob.left(((sides -2)*180)/sides)

My problem is with the last line. I'm pretty confident of the maths being correct because the formula for the total interior angles for a regular polygon is (n-2)*180 where 'n' is the number of sides. And then to get each individual corner angle you then divide the result of the last formula with n again.

But I'm not getting the result I should. I'm wondering if there's a syntax error with how I've laid out the arithmetic??
Any help is appreciated