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