I'm trying to create plots of Fourier Synthesis and FFT Spectrum of a square wave using harmonics, but I keep getting a vector length error for the FFT spectrum.

N = 5;
f0 = 5;
tw = 1;

k = [1:2:N];
ak = 2./(pi*k);

T0 = 1/f0;            
fs = 20*2*N*f0;         
ts = 1/fs;
                        
y = 0.5;


for m=1:length(k)
    t=(0:ts:tw);
    x = ak(m).*cos(2*pi*f0*k(m)*t-pi/2);
    y = y+x;
    h = plot(t,y);
    xlabel('Time (s)') 
end


fa = 0:fs/N:fs-fs/N;
f = fft(y)/N;
F = abs(f);
figure, plot(fa,f)