help with expressions!!

[CyCLoBoT]

I would appreciate it if someone can help me with this problem. A user enters an expression like A + B / (C+D). I am suppose to ask the user for the values of A, B, C and D and I have to output the prefix notation of that expression and the answer of that sum.

When the user inputs A + B / (C+D) where a= 1, b=2,c=3 and d = 4

The output would be:
+ A / B +CD

and the sum : 1+2/(3+4) = 4/7

The expression can have a maximum of 4 variables.

Thanks