现有Python turtle代码如下，执行后输出对应阶梯图形，请补全第4行range后的参数：

import turtle

turtle.pensize(3)
for i in range(________):
    if i%2==0:
        turtle.pencolor("red")
        turtle.forward(40)
        turtle.left(90)
    else:
        turtle.pencolor("blue")
        turtle.forward(20)
        turtle.right(90)