python畫一組同心圓

import turtle as a
m = eval(input())#圓的半徑
n = eval(input())#圓的個數
c = input()#畫筆的顏色
a.pencolor("{}".format(c))
for i in range(n):
    a.penup()
    a.goto(0, -20 * i)
    a.pendown()
    a.circle(m)
    m+=20
a.hideturtle()
a.done()