Add As I Go

Usin EXCEL for WIN 98 I want to keep a running total of the figures I put in a column. EX: In col A1 I insert 5 in A2 6, in cell b2 I want the result 11. Then in cell a3 I put in 4, b3 should = 15, in a4 I put 5, b4 should be 20. How can I copy the formula =SUM(a1+a2) so that it adds a3 a4 a5…..a256 as I enter figures?