excel vba programmer

in column a i have some numeric value for example value is 100 its updating live in every 2 minutes with new value might it will be 90 as new value. so copied the 1st updated value 100 in next column assign formal =a2+b2 so value will be 190 but next 2 minute the update value is in column a is 35 so its need to add 190+35 but unfortunately it's adding 35 +100 not incrementing with new value. can any 1 help what formula need to use.