"RPN" or "Reverse Polish Notation" was popular in calculators of the 1970s. (Not so much now.) It works like this: Instead of typing "1 + 2", you type "1 2 +". The advantage is that you don't need parentheses. Instead of "(1 + 2) * (3 + 4)", you type "1 2 + 3 4 + *". The idea is that each number gets put on a "stack" of intermediate results. When you see an operator (+, -, *, or /), you take the top two numbers off the stack and apply the operator, and put the result back on the stack. The function RPN, giving a string of numbers and operators (all separated by spaces), should return the numeric value of the expression.