Afl code to add and display summation result ? Help needed

Hi ,

I am learning afl coding , as an excercise i am stuck in one example , please help me to solve

B is the result , its just summation of total a values
i.e B=a+a(n-1)+a(n-2)


Slno A B FORMULA USED TO GET B
1 5 15 a1+b2
2 4 10 a2+b3
3 3 6 a3+b4
4 2 3 a4+b5
5 1 1 a5+0