var a:array[1..9000] of 0..7100;
    v:array[1..7100] of word;
    fr:array[1..7100] of word;
    f:text;
    n,i,j,db:word;
    z:longint;
begin
  assign(f,'medie.in');
  reset(f);
  readln(f,n);
  for i:=1 to n do readln(f,a[i]);
  close(f);
  for i:=1 to n-1 do begin
    fr[a[i]]:=fr[a[i]]+1;
    for j:=i+1 to n do begin
         z:=(a[i]+a[j]) div 2;
      if (a[i]+a[j]) mod 2 = 0 then v[z]:=v[z]+1;
    end;
  end;
  i:=i+1;
  fr[a[i]]:=fr[a[i]]+1;

  for i:=1 to n do
    if v[a[i]]<>0 then begin
      db:=db+v[a[i]]-fr[a[i]]+1;
    end;
 
 assign(f,'medie.out');rewrite(f);
 writeln(f,db);
 close(f);
end.