C / C++ / MFC

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

using namespace std;

int sum = 0;
int ssum = 0;
int msum = 0;
double dsum = 0;
int x = 0;
int y = 0;
int menu();
int c = menu();

int main()
{	
	do
	{
		menu();

		if(c == 1)
			 add();
		else if(c == 2)	
			 sub();
		else if(c == 3)
			 multi();
		else if(c == 4)
			 div();
	}

	while(c == 5);
		cout<<"Thank you for using the calculator!"<<endl;
		
	getch();
	return 0;
}
		 
menu()
{	
	int sel = 0;

    cout<<"Please select a menu choice..."<<endl
    <<"1. Add"<<endl
    <<"2. Subtract"<<endl
    <<"3. Multiply"<<endl
    <<"4. Divide"<<endl
    <<"5. Exit program"<<endl;
		cin>>sel;
		
	if(c > 5 && c < 1)
		cout<<"You have entered an invalid selection!"<<endl;

	return sel;
}

int add()
{
        cout<<"Please enter a number..."<<endl;
		cin>> x;
		cout<<"Please enter your second number..."<<endl;
		cin>>y;

		sum = x + y;
		cout<<"The answer is "<<sum<<endl;

	    return sum;
}

int sub()
{
        cout<<"Please enter a number..."<<endl;
		cin>> x;
		cout<<"Please enter your second number..."<<endl;
		cin>>y;
	
		ssum = x - y;
		cout<<"The answer is "<<ssum<<endl;

	    return ssum;
}

int multi()
{
		cout<<"Please enter a number..."<<endl;
		cin>> x;
		cout<<"Please enter your second number..."<<endl;
		cin>>y;
	
		msum = x * y;
		cout<<"The answer is "<<msum<<endl;

		return msum;
}

double div()
{
		cout<<"Please enter a number..."<<endl;
		cin>> x;
		cout<<"Please enter your second number..."<<endl;
		cin>>y;
	
		dsum = x / y;
		cout<<"The answer is "<<dsum<<endl;

		return dsum;
}

--------------------Configuration: assign4 - Win32 Debug--------------------
Compiling...
assign4.cpp
D:\Assignment 4\assign4.cpp(33) : error C2065: 'add' : undeclared identifier
D:\Assignment 4\assign4.cpp(35) : error C2065: 'sub' : undeclared identifier
D:\Assignment 4\assign4.cpp(37) : error C2065: 'multi' : undeclared identifier
D:\Assignment 4\assign4.cpp(39) : error C2660: 'div' : function does not take 0 parameters
D:\Assignment 4\assign4.cpp(68) : error C2373: 'add' : redefinition; different type modifiers
D:\Assignment 4\assign4.cpp(81) : error C2373: 'sub' : redefinition; different type modifiers
D:\Assignment 4\assign4.cpp(94) : error C2373: 'multi' : redefinition; different type modifiers
Error executing cl.exe.

assign4.obj - 7 error(s), 0 warning(s)