sum of value in another field

hi all!!!

I would like to know if there is a possible way to add up a single field in another field. for e.g

if I have a decimal field 'A' and a decimal field 'B'. If 'A' has value 30 in it 'B' should also have the value 30, if I overwrite 'A' with 50 it should sum up 30 and 50 and field 'B' should show the value 80 in it. Basically, it should sum all the values that are being written in the field 'A'.

Thanks in advance.