C++ Function Parameter Passing a variable to a function

#include <iostream> 
#include <math.h> 

using namespace std; 

int main() /*from  w  w w.  jav  a  2 s  .c o m*/
{ 
  double start; 
  double finish; 

  start = -3.5; 
  finish = fabs(start); 

  cout << finish << endl; 
  return 0; 
}