series equation?!

I am not mainly asking about a code but I would like to have

some help with how to start logically with solving this series:

II=4*(1-1/3+1/5-1/7+1/9-1/11+1/13-.....)

the program will ask the user about the number of terms he

wants,

and then depend on these terms calculate and display the

approxmation of II.

for ( i = 1 ; i <= 13 ; i += 2 )
That generates all the denominators for your terms in your series

int sign = 1;
sign = sign * -1;
If you keep doing that in a loop, sign alternates between +1 and -1
Use this to alternately add and subtract successive terms

many thanks for your great help.