function [atil b]=contorno2(nv,a,n,m)
atil=a;
b=-ones(nv,1);
for i=1:n
  for j=1:m
    if ((i==1 || i==n) || (j==1 || j==n) || (abs(j-20)<4 && abs(i-25)<4))
          k=(j-1)*n + i;
          atil(k,:)=0;
          atil(k,k)=1;
          b(k)=0;
    end
  end
end