Clicker game counter help

[JohnNo]

from tkinter import *

root = Tk()
root.geometry("200x200")
root.title("My Clicker Game")

global counter
counter = 0

def nClick():
    global counter
    counter += 1
    mLabel.config(text = counter)

mButton1 = Button(text = counter, command = nClick, fg = "darkgreen", bg = "white")
mButton1.pack()
mButton2 = Button(text = "Goodbye", command = root.destroy, fg = "darkblue", bg = "white")
mButton2.pack()
mLabel = Label()
mLabel.pack()

root.mainloop()

I need something like if counter is 20 then program will do something but I dont know how to do that I have tried this.

if counter == 20:
   root.destroy

but that does not work so I ask if someone can help