function [y] = ackley( xx )
    d = 2;
    a = 20;
    b = 0.2;
    c = 2.*pi;
    s1 = xx(:,1).^2 + xx(:,2).^2;
    s2 = cos(c*xx(:,1)) + cos(c*xx(:,2));
    y = -a.*exp(-b.*sqrt(1./d.*s1))-exp(1./d.*s2)+a+exp(1);
end