As I'm not a guru yet in C++, here are few pointers

 - Function names should start with lowercase letters e.g `Minus()` should be `minus`
 - There is no need to have `int` has a return type for your function. `void` is a valid return type and it means return nothing. This would save you the lines of `return 0`
 - Your `divide` and `multiply` functions may be prone to erroneous output. This casts your answer to int
 - Avoid `using namespace std` because when you use additional libraries clashes may occur. Instead use qualified names e.g std::cout, std::endl etc
 - I see duplicate codes, DRY up your main function. Make first and second numbers global so this can be read once and it's outside your if statement 
 - At this point, I will opt for a `switch` statement to make your `main` cleaner
 - To make this robust, you probably have to check for if invalid input was typed by the user