function E=twos(a,x,y)
x=x(:);y=y(:);
% Y=1/(sqrt(2*pi)*a(1))*exp(-(x-a(2))^2/(2*a(1)^2))+1/(sqrt(2*pi)*a(3))*exp(-(x-a(4))^2/(2*a(3)^2));
% Y=1/(sqrt(2*3.1415926)*a(1))*exp(-(x-a(2))^2/(2*a(1)^2))
Y=a(1)*exp(-a(3)*x)+a(2)*exp(-a(4)*x);
E=y-Y;