var f1,f2:text; i,j,caut,n:longint;
    x,v,a:array[-10000..10000] of longint;
    nr:qword;
begin
        assign(f1,'medie.in');
        reset(f1);
        assign(f2,'medie.out');
        rewrite(f2);
        read(f1,n);
        for i:=1 to n do
        begin
                read(f1,a[i]);
                inc(x[a[i]]);
        end;
        for i:=1 to n do
        begin
                v:=x;
                caut:=2*a[i];
                dec(v[a[i]]);
                for j:=1 to n do
                begin
                        if j<>i then
                        begin
                                dec(v[a[j]]);
                                nr:=nr+v[caut-a[j]];
                        end;
                end;
        end;
        writeln(f2,caut);
        close(f1);
        close(f2);
end.