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

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(_________ + ____________);
}