var ap:array[0..7000] of longint;
    a,v:array[0..9000] of longint;
    f,g:text;
    i,j,n,sol:longint;
begin
 assign(f,'medie.in'); reset(f);
 assign(g,'medie.out'); rewrite(g);
 read(f,n);
 for i:=1 to n do
  read(f,a[i]);
 for i:=1 to n-1 do begin
  inc(ap[a[i]]);
  for j:=i+1 to n do
   if (a[i]+a[j]) and 1=0 then
    inc(v[(a[i]+a[j]) shr 1]);
 end;
 inc(ap[a[n]]);
 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.