#include <stdio.h>
#include <stdlib.h>

unsigned long long fib_iter(unsigned long long a)
{
  unsigned long long r = 0;
  unsigned long long minus_one = 1;
  unsigned long long minus_two = 0;
  unsigned long long i;

  if (a == 0) {
    r = 0;
  } else if (a == 1) {
    r = 1;
  } else {
    for (i = 2; i <= a; i++) {
      r = minus_one + minus_two;
      minus_two = minus_one;
      minus_one = r;
    }
  }

  return r;
}

int main(int argc, char *argv[])
{
  unsigned long long a = 0;
  unsigned long long r = 0;

  a = atol(argv[1]);
  r = fib_iter(a);
  printf("fib(%llu) = %llu\n",a,r);
  return 0;
}