type integer=longint;
var v:array[1..10000] of integer;
    s:array[1..10000] of integer;
    n,i,j,k,r,m:integer;
begin
  assign(input,'medie.in'); reset(input);
  assign(output,'medie.out'); rewrite(output);
  readln(n);
  for i:=1 to n do begin
    readln(v[i]); inc(s[v[i]]);
  end;
  for j:=1 to n-1 do
    for k:=j+1 to n do
      if (v[j]-v[k]) mod 2=0 then begin
        m:=(v[j]+v[k]) div 2;
        if v[j]=v[k] then inc(r,s[m]-2)
        else inc(r,s[m]);
      end;
  writeln(r);
  close(input); close(output);
end.