type vect = array[1..805] of integer;

var fi,fo:text;
    i,j,n,x,y,z,mj,st,dr:integer;
    a:vect;
    ntri:longint;

 procedure heapsort(n:integer);
  var aux,k:integer;
  begin
   for i:=1 to n do
    begin
     j:=i;
     while (j div 2<>0) and (a[j div 2]<a[j]) do
      begin
        aux:=a[j div 2];
        a[j div 2]:=a[j];
        a[j]:=aux;
        j:=j div 2;
      end;
    end;
   i:=n;
   while i>1 do
    begin
     aux:=a[1];
     a[1]:=a[i];
     a[i]:=aux;
     dec(i);
     j:=1;
     while (1>0) do
      begin
       k:=2*j;
       if (k>i) then  break;
       if (k+1<=i) and (a[k+1]>a[k]) then inc(k);
       if a[j]>=a[k] then break;

       aux:=a[j];
       a[j]:=a[k];
       a[k]:=aux;
       j:=k;
      end;
     end;
  end;





begin
 ntri:=0;
 assign(fi,'nrtri.in'); reset(fi);
 assign(fo,'nrtri.out'); rewrite(fo);
 readln(fi,n);
 for  i:=1 to n do
  read(fi,a[i]);
 heapsort(n);
 for i:=1 to n-2 do
  for j:=i+1 to n-1 do
   begin
    x:=a[i];
    y:=a[j];
    st:=j+1;
    dr:=n;
    z:=x+y;
    while st<=dr do
     begin
      mj:=(st+dr) div 2;
      if z<a[mj] then dr:=mj-1;
      if z>a[mj] then st:=mj+1;
      if z=a[mj] then break;
     end;
    while (a[mj]<z) and (mj<n) and (a[mj+1]<=z) do
     inc(mj);
    while (a[mj]>z) and (mj>j) do
     dec(mj);
    if a[mj]=z then
     begin
      while a[mj]=z do
       inc(mj);
      dec(mj);
     end;
    if a[mj]<=z then  inc(ntri,mj-j);
   end;
writeln(fo,ntri);
close(fo);
end.