Write a function mean, which follows the following specification:

Input: a$0,$ a$1,$ a$2,$ and a$3$ contain $($signed$)$ integers, with

Output: Return in a$0$ the arithmetic mean of the input values. If the mean is not an integer, round the number down.

Example: If a$0$ contains $1,$ a$1$ contains $2,$ a$2$ contains $3,$ and a$3$ contains $4,$ then the mean is

$.$ Thus, the expected output is $2.$

Example: If a$0$ contains $-1,$ a$1$ contains $-2,$ a$2$ contains $-3,$ and a$3$ contains $-4,$ then the mean is

$.$ Thus, the expected output is $-3.$

Hint: RISC$-$V standard does not have a division instruction or a multiplication instruction. However, some other instruction can be used to divide by certain numbers. What instruction is that?