public static long itFibN(int n)
{
 if (n < 2)
  return n;
 long ans = 0;
 long n1 = 0;
 long n2 = 1;
 for(n--; n > 0; n--)
 {
  ans = n1 + n2;
  n1 = n2;
  n2 = ans;
 }
 return ans;
}