program medie;
var f, g:text;
    i, j, n, x, max, rez:longint;
    v:array[1..7000] of longint;

procedure citire;
  begin
    readln(f,n);
    for i:=1 to n do
      begin
        read(f,x);
        v[x]:=v[x]+1;
        if x>max then
          max:=x;
      end;
  end;

procedure rezolvare;
  begin
    for i:=1 to max do
      if v[i]>0 then
        begin
          for j:=1 to i-1 do
            rez:=rez+v[j]*v[2*i-j];
          if v[i]>2 then
            rez:=rez+(v[i]*(v[i]-1)*(v[i]-2)) div 2;
        end;
    writeln(g,rez);
  end;

  begin
    assign(f,'medie.in'); reset(f);
    assign(g,'medie.out'); rewrite(g);
    citire;
    rezolvare;
    close(f);
    close(g);
  end.