can't solve an exercise-help me with it

[hossam]

I'm studying python newly and have an exercise that is difficult for me as a 
beginner.Here it is :
"Write a program that approximates the value of pi by summing the terms of 
this series:
4/1-4/3+4/5-4/7+4/9-4/11+.... The program should prompt the user for n, the 
number of terms to sum and then output the sum of the first n terms of this 
series."

any help would be appreciated.

benni