fib.c

#include <stdio.h>
#include <math.h>

double fib(int n){
  double a = 1;
  double b = 1;
  while(--n > 0){
    b += a;
    a = b - a;
  }
  return b;
}
  
int main(int argc, char *argv[]){
  printf("Hello, World!");
  int n = 5;
  double a = fib(n);
  printf("fib(%d) = %lf", n, a);
  return 0;
}