function t=exam1_3(xn,N)
%this function calculate the elapsed seconds of
%the function of fft and ditfft.
%the argument 'xn'is the sample sequence input.
%the argument 'N'is the length of the sample sequence input.
%N=32768/2; xn=[0 1 2 3 4 5 6 7];
if length(xn)<N    
    xn=[xn,zeros(1,N-length(xn))];    
end
yn=xn;
tic
fft(xn,N);
t(1)=toc;
tic
ditfft(yn,N);
t(2)=toc;