const NMAX=30002;
var n,i,j,k:integer;
    v:array[0..801] of integer;
    nr:array[0..NMAX+1]of integer;
    f,g:text;
    sol:longint;
begin
 assign(f,'nrtri.in');reset(f);
 assign(g,'nrtri.out');rewrite(g);
 readln(f,n);
 for i:=1 to n do
  begin
  read(f,v[i]);
  inc(nr[v[i]]);
  end;
 for i:=1 to NMAX do
  nr[i]:=nr[i]+nr[i-1];
 for i:=1 to n do
  for j:=i+1 to n do
   if (v[i]>v[j]) then
    begin
    k:=v[i];
    v[i]:=v[j];
    v[j]:=k;
    end;
 sol:=0;
 for i:=1 to n-1 do
  for j:=i+1 to n do
   begin
   k:=v[i]+v[j];
   if k>NMAX then k:=NMAX;
   sol:=sol+(nr[k]-nr[v[j]-1]-1);
  end;
 write(g,sol);
 close(f);close(g);
end.