Площадь треугольника по формуле Герона

from random import randrange as rnd
from tkinter import *

root = Tk()
root.geometry('600x600')
canv = Canvas(bg='white')
canv.pack(fill=BOTH, expand=1)

x1 = None
y1 = None
x2 = None
y2 = None
x3 = None
y3 = None


def geron(x1,y1,x2,y2,x3,y3):
    a12 = ((x2-x1)**2+ (y2-y1)**2)**0.5
    a23 = ((x2-x3)**2+ (y2-y3)**2)**0.5
    a13 = ((x3-x1)**2+ (y3-y1)**2)**0.5
    p = (a12 + a23 + a13)/2
    return (p*(p-a12)*(p-a23)*(p-a13))**0.5

def click(event):
    global x1,x2,x3,y1,y2,y3
    r = 3
    x = event.x
    y = event.y
    if x1 is None:
        x1 = x
        y1 = y
    elif x2 is None:
        x2 = x
        y2 = y
    elif x3 is None:
        x3 = x
        y3 = y
        canv.create_polygon(x1,y1,x2,y2,x3,y3,x1,y1, fill='orange')
        s = geron(x1,y1,x2,y2,x3,y3)
        print(s)
        canv.create_text(300,300,text=s,fill='gray',font='Tahoma 30')
    else:
        canv.delete(ALL)
        x1 = x
        y1 = y
        x2 = None
        y2 = None
        x3 = None
        y3 = None

    canv.create_oval(x-r,y-r,x+r,y+r,fill='orange')

root.bind('<1>',click)

mainloop()