Pascal :

var n,i,j,ans:longint;

    a:array[0..1001]of longint;

    vis:array[0..20001]of boolean;

begin

 readln(n);

 for i:=1 to n do

  begin

   read(a[i]);

   vis[a[i]]:=true;

  end;

 for i:=1 to n do

  for j:=1 to i-1 do

   if vis[a[i]+a[j]] then

    begin

     inc(ans);

     vis[a[i]+a[j]]:=false;

    end;

 write(ans);

end.