Fibonacci series using a recursive function

Hello all :) this my first post, so bear with me if I don't do something correctly.:eek:

This is my question: below I have the format for my recursive function but am really unsure of what I need to put in the Underscore areas ?

If anyone has any ideas, I have never done a Fibonacci series of numbers before, so I'm lost as to how it has to be set up.

#include <iostream>

using namespace std;

unsigned long fib(int nth);

void main()

{

int nth=0;

cout << "Enter an integer from 1 to 30: ";

while (!(cin >> nth))

{

cin.clear();

while (cin.get() != '\n')

continue;

cout << " Please! Enter a number: ";

}

if((nth < 1) || (nth > 30))

cout << "\nSorry. "

<< "Number isn't in the right range.";

else

{

unsigned long fibValue;

fib(int fibValue);

cout << "\nThe value of number " << nth;

cout << " in the series is " << fibValue

<< ".\n";

}

}

// Recursive function

unsigned long fib(int nth)

{

if(______)

return(___);

else

return(_________ + ____________);

}