Apply the formula
double num = ( 1 - Math . Sqrt ( 5 ) ) / 2 ; double val = Math . Round ( Math . Abs ( 1 / ( Math . Pow ( num , N + 2 ) + Math . Pow ( num , N + 1 ) + Math . Pow ( num , N ) + Math . Pow ( num , N - 1 ) ) ) - 1 ) ;