function fibb(n) {
  if (n === 0) return 0;
  if (n === 1) return 1;
  
  let prev = 0;
  let current = 1;
  
  for (let i = 2; i <= n; i++) {
    let next = prev + current;
    prev = current;
    current = next;
  }
  
  return current;
}