I wish to have a computer whose machine$-$level instructions are all $32$ bits each. If I want to have all instructions of the form add R$1,$ R$2,$ R$3,$ where R$1,$ R$2$ and R$3$ are registers. What is the maximum number of instructions that my machine can have, if my computer needs to have at least $15$ different registers referenced in the instructions? $($Put $\text{'}0\text{'}$ if not possible.$)$