编写函数fun,函数的功能是：根据以下公式计算s,计算结果作为函数值返回；n通过形参传入。

s=1 + 1/(1+2) + 1/(1+2+3) +... + 1/(1+2+...+n)

-Write a function fun, function is the function: according to the following formula s, calculated as a function return value　n passing through the parameter. s = 1+ 1/(1+2)+ 1/(1+2+3)+ ...+ 1/(1+2+...+ n)