If you start with k cans in the fridge, you take one there is now k-1 cans in the fridge, you add back two, according to the rule, there are now k+1 can in the

If you start with k cans in the fridge, you take one there is now k-1 cans in the fridge, you add back two, according to the rule, there are now k+1 can in the fridge. You and your colleagues repeat this process n times, n being the number of total cans drank. At the end of the repeating n times there are k+n cans in the fridge, and n cans in the garbage bin.