Fourier Series part wise function.

I have a part wise function which im trying to plot with its Fourier series. This is my code, since my Fourier has coefficients I have to find them and then do the sum but im not getting the plot im supposed to. What am I doing wrong? NOTE: the first loop its for me to plot the original function.

J = 100; x = linspace(-pi,pi,J); M = 20;

for k = 1:length(x)

if (x(k) >= -pi && x(k) <= 0)

    f(k) = (1 + (x(k)/pi));

elseif (x(k) >= 0 && x(k) < 0.5*pi)

    f(k) = (1 - (2*x(k)/pi));

else (x(k) > 0.5*pi && x(k) <= pi);

    f(k) = (-1 + (x(k)/pi));

end

end

 for n = 1:M

    A_0 = 5/8;
    A_n = 1/n/pi*(sin((n*pi)/2)/2+3/n/pi*(1-cos((n*pi)/2)));
    B_n = -1/n/pi*(cos((n*pi)/2)/2)+(3/n/pi)*sin((n*pi)/2);
    FS = A_0/2 + A_n*cos(n*x) + B_n*sin(n*x);

end

 plot(x,FS,'r')
 hold on
 plot(x,f)

Thanks in Advance.