how to define function prototypes in c++

/* C++ Function Prototype and C++ Function Definition */

#include<iostream.h>
#include<conio.h>
#include<stdlib.h>

int add(int, int);         // function prototype
int subtract(int, int);    // function prototype
int multiply(int, int);    // function prototype
int divide(int, int);      // function prototype

void main()
{
	clrscr();
	int a, b;
	cout<<"Enter any two number: ";
	cin>>a>>b;
	cout<<"\nSummation = "<<add(a, b);
	cout<<"\nSubtraction = "<<subtract(a, b);  
	cout<<"\nMultiplication = "<<multiply(a, b); 
	cout<<"\nDivision = "<<divide(a, b); 
	getch();
}

int add(int x, int y)     // function definition
{
	int res;
	res = x + y;
	return res;
}

int subtract(int x, int y)    // function definition
{
	int res;
	res = x - y;
	return res;
}

int multiply(int x, int y)    // function definition
{
	int res;
	res = x * y;
	return res;
}

int divide(int x, int y)    // function definition
{
	if(y==0)
	{
		cout<<"\n\nDivide by Zero Error..!!";
		cout<<"\nPress any key to exit...";
		getch();
		exit(1);
	}
	else
	{
		int res;
		res = x / y;
		return res;
	}
}