问题描述:
Write a python function to count the maximum number of equilateral triangles that can be formed within a given equilateral triangle.

测试用例:
['assert No_of_Triangle(4,2) == 7', 'assert No_of_Triangle(4,3) == 3', 'assert No_of_Triangle(1,3) == -1']

生成的代码:
def No_of_Triangle(n, k):
    if k > n:
        return -1
    return (n - k + 1) * (n - k + 2) // 2