function interface=interface(L,N)
%L=lattice size, N=# of MC steps

A=ones(L,L);
if mod(L,2)==0
    mean(1,1)=1;
else 
    mean(1,1)=-1;
end

%set right-hand side to -1
for u=1:L
    for v=L/2+1:L
        A(u,v)=-1;
    end
end

for step=2:N
%pick random site
i=ceil(L*rand(1));
j=ceil(L*rand(1));

%pick 1 of 4 nearest neighbors
nn=ceil(4*rand(1));

%get indices of nn
if nn==1 %exchange top
    inn=ndx(i-1,L);
    jnn=ndx(j,L);
elseif nn==2 %exchange bottom
    inn=ndx(i+1,L);
    jnn=ndx(j,L);
elseif nn==3 %exchange left
    inn=ndx(i,L);
    jnn=ndx(j-1,L);
elseif nn==4 %exchange right
    inn=ndx(i,L);
    jnn=ndx(j+1,L);
end

%exchange if different
if A(i,j)~=A(inn,jnn)
    A(i,j)=A(inn,jnn);
    A(inn,jnn)=-A(i,j);
    sumold=0;
    %get mean magnetization on one side of the interface
    for i=1:L
        sumnew=A(i,L/2)+sumold;
        sumold=sumnew;
    end
    mean(step,1)=sumold/L;
end

end

figure(1)
plot(mean)