Volume · Challenge problem

Leo builds a staircase out of $1$-foot cubes. It has $4$ steps and is $3$ feet wide. The first step is $1$ cube tall, the second step is $2$ cubes tall, the third is $3$ cubes tall, and the fourth is $4$ cubes tall. Each step is $1$ foot deep. What is the volume of the staircase, in cubic feet?