This is my problem

I have only two cells in the excel say A1 and B1.

I want the sum of the values in the cell A1 to be displayed in B1.

For Eg: I enter 10 for the first time in the cell A1, I want 10 to be
displayed in B1.
The second time I enter 20 in the cell A1, I want 30(20+10) to
be displayed in B1.
The third time I enter 30 in the cell A1, I want 60(20+10+30)
to be displayed in B1.

Thus 30 should be displayed in the cell A1 and 60 should be displayed in the
cell B1.

i.e the latest value entered should be displayed in A1 and the cumulative
sum of the values entered should be displayed in B1.

Please help me out!