var ap:array[0..7000] of longint;
    a,v:array[0..9000] of longint;
    f,g:text;
    i,j,n,sol:longint;
    aux:longint;

begin
 assign(f,'medie.in'); reset(f);
 assign(g,'medie.out'); rewrite(g);
 read(f,n);
 for i:=1 to n do begin
  read(f,a[i]);
  ap[a[i]]:=ap[a[i]]+1;
 end;
 for i:=1 to n-1 do
  for j:=i+1 to n do begin
   aux:=a[i]+a[j];
   if aux and 1=0 then
    inc(v[aux shr 1]);
  end;
 sol:=0;
 for i:=1 to n do
  if v[a[i]]<>0 then
   sol:=sol+v[a[i]]-ap[a[i]]+1;
 writeln(g,sol);
 close(f); close(g);
end.