var v:array[1..7000]of integer;
    k,s,i,j,n,nr:longint;
begin
assign(input,'medie.in');reset(input);
assign(output,'medie.out');rewrite(output);
read(n);
 for i:=1 to n do begin
  readln(nr);
  inc(v[nr]);
 end;
 for i:=1 to n do
  if v[i]>2 then s:=s+((v[i]-2)*n*(n-1)div 2);
 k:=7000;
 while v[k]<>0 do dec(k);
 for i:=1 to k do
  for j:=i+1 to k do
   if abs(v[i]-v[j])mod 2=0 then begin inc(s,v[(i+j)div 2]);v[(i+j)div 2]:=0;end;

writeln(s);
close(input);close(output);
end.