program medi;
var f:text;
    i,n,j,nr:longint;
    v:array[1..9000] of integer;
    a:array[1..7000] of integer;
    viz:array[1..7000] of integer;
begin
  assign(f,'medie.in');
  reset(f);
  readln(f,n);
  for i:=1 to n do begin
                    readln(f,v[i]);
                    viz[v[i]]:=viz[v[i]]+1;
                   end;
  close(f);
  nr:=0;
  for i:=1 to n-1 do
    for j:=i+1 to n do
      if (v[i]<>v[j]) and((v[i]+v[j])mod 2=0) then a[(v[i]+v[j])div 2]:=a[(v[i]+v[j])div 2]+1;
  for i:=1 to 7000 do
   if v[i]=1 then nr:=nr+a[i]
             else if v[i]>1 then nr:=nr+a[i]*viz[i]+viz[i]*(viz[i]-1)*(viz[i]-2)div 2;
  assign(f,'medie.out');
  rewrite(f);
  writeln(f,nr);
  close(f);
end.