I want to build a library

I have written this program.

        #include "stdafx.h"
        #include"iostream"
        #include<conio.h>
        using namespace std;

        int S(int a, int b){
            return a + b;
        }
        int M(int a, int b){
            return a * b;
        }

    int main()
    {
        int a, b,c,d;
        a = 2;
        b = 3;
        c=S(a, b);
        d=M(a, b);
        cout << c <<endl<< d;
        _getch();
        return 0;
    }

I want to define a library that includes functions S and M. I want this program to be written as follows. can you help me?