function y = f_bar(x1,x2)

b = [1 1 1 1 1 1]';
A = [0.4873 -0.8732
     0.6072  0.7946
     0.9880 -0.1546
    -0.2142 -0.9768
    -0.9871 -0.1601
     0.9124  0.4093];

if sum(b-A*[x1;x2]<0) == 0
    y = -sum(log(b-A*[x1;x2]));
else
    y =NaN;
end

end