assume cell a1 is current number of contracts for the current minute

asume cell b1 is the previous number of contracts for the previous minute

when the seconds (1 - 59) roll over to 1 again, the a1 number of contracts rolls into b1 and a1 turns to 0. How do I keep a running total of all contracts sold? When I use =sum(b1 + a1) is is wrong because the previous number that was in b1 is replaced by the number that was in a1. I need to keep track of the contracts regardless of how many minutes i run the routine for