program medie;
var f,g:text;
    v:array[0..100000]of longint;
    n,i,x,m,j:longint;
    c,s:int64;
begin
assign(f,'medie.in');
assign(g,'medie.out');
reset(f);
rewrite(g);
read(f,n);
for i:=1 to n do
  begin
    read(f,x);
    inc(v[x]);
    if (x>m)then m:=x;
  end;
for i:=1 to m do
  begin
    s:=0;
    for j:=1 to v[i]-2 do inc(s,j);
    s:=v[i]*s;
    inc(c,s);
    s:=0;
    if (v[i]>0)then
      for j:=1 to i-1 do
        inc(s,v[j]*v[2*i-j]);
    inc(c,v[i]*s);
  end;
write(g,c);
close(f);
close(g);
end.