ECE - logic circuit for a 4 bit input which counts 1s in input

[physstudent1]

Homework Statement

Using basic gates and/or building blocks, design a logic circuit that takes in a 4 bit input and will compute the number of 1s in the input.

Homework Equations

The Attempt at a Solution

I have no idea where to start can anyone help please.

 

[wildman]

Maybe a good place to start is to enumerate your basic building blocks telling us what each one does... That will get you started.