#include <stdio.h>

int main()
{
  unsigned int num;
  unsigned int i, fn, fn_1 = 1, fn_2 = 0;

  printf("Enter a non-negative integer: ");
  if (scanf("%u", &num) != 1) {
    printf("ERROR: failed to read input number!\n");
    return -1;
  }
  if (num == 0)
    fn = 0;
  else if (num == 1)
    fn = 1;
  else {
    for (i = 2; i <= num; ++i) {
      fn = fn_2 + fn_1;
      fn_2 = fn_1;
      fn_1 = fn;
    }
  }
  printf("Fibonacci of %d is %d\n", num, fn);
  return 0;
}