Excel Calculation

I

Ian

I want to calculate a figure in a cell as follows
cell d1 = 100 cell e1 = cell d1 plus any subsequent number
input reduced ie
d1 = 100 e1 = 100
I input 10 in d1 e1 now = 90
I input 20 in d1 e1 now = 70
I input another 20 in d1 e1 now = 50

Is there a formula that can do this?
 
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