I suspect it's this bit:
Operand2 = Input;
Which you do twice, but for the first operand you do this:
Operand1 = Operand1 + Input;
which would make more sense for both of them.
But your error description is very vague, and I can't be sure what problem with that code you are seeing - I see quite a few, but you're a beginner, so most of them are kinda excusable "why on earth are you doing it like that?" moments.
Start by debugging your code - you have a tool available to you which will help you find out what is going on: the debugger. If you don't know how to use it then a quick Google for "Visual Studio debugger" should give you the info you need.
Put a breakpoint on the first line in the function, and run your code through the debugger. Then look at your code, and at your data and work out what should happen manually. Then single step each line checking that what you expected to happen is exactly what did. When it isn't, that's when you have a problem, and you can back-track (or run it again and look more closely) to find out why.
Sorry, but we can't do that for you - time for you to learn a new (and very, very useful) skill: debugging!