It's really simple + and - only but you can take it as an example. You could also make it so that you can exit anytime to the snake os menu. You just need to add * and / .
like this...:
@A
INPUT"FIRST #";A
INPUT"SECOND #";B
INPUT"SIGN";A$
IF A$=="-"THEN PRINT A-B
IF A$=="×" THEN PRINT A*B
IF A$=="+" THEN PRINT A+B
IF A$=="÷" THEN PRINT A/B
GOTO @A
Hey you should consider a REAL calculator
If you go to "Reverse Polish Notation" on wikipedia then it tells you how to convert an expression into a form thats easy to calculate
It could easily solve things like
"90/4.6*0.1/(3-123)*(7/(4-8.45776))/(2^3-1)/3)"