Water

First of all, we will generate all possible combinations of “$a$”($0.5$-liter bottle) and “$b$”($1$-liter bottle) by a nested loop and count the sum of liters of every combination in $O(a\times b)$ complexity.
Now for each $i$ from $0$ to $c$, we will try to take $i$ number of $2$ liters bottle. That is to say, for each $i$, we will add the count of $(n-2\times i)$ liters generated by the $a,b$ combination to the answer.