picomath

using System;

class Phi {

    public static double phi(double x)
    {
        // constants
        double a1 = 0.254829592;
        double a2 = -0.284496736;
        double a3 = 1.421413741;
        double a4 = -1.453152027;
        double a5 = 1.061405429;
        double p = 0.3275911;
            
        // Save the sign of x
        int sign = 1;
        if (x < 0)
            sign = -1;
        x = Math.Abs(x) / Math.Sqrt(2.0);
            
        // A&amp;S formula 7.1.26
        double t = 1.0 / (1.0 + p*x);
        double y = 1.0 - (((((a5*t + a4)*t) + a3)*t + a2)*t + a1)*t * Math.Exp(-x*x);
            
        return 0.5 * (1.0 + sign*y);
    }

}