function y=df(x)
%y=1-log(10)*10.^x;
%y=4*x^3+15*x^2-18*x-85;
%y=2400*x^3-1650*x^2+400*x-20;           %y=600*x^4-550*x^3+200*x^2-20*x-1
%y=1-cos(x);                             %y=x-sin(x)
%y=(1-cos(x))^2-(x-sin(x))*sin(x);
y=exp(-x^2/2)/(2*pi)^0.5;