{medie ONI2006}

var v:array[1..9000] of integer;
    aparitie,media:array[1..7000] of integer;
    n:integer;
    suma:longint;
    f,g:text;

procedure citire;
var i:integer;
begin
assign(f,'medie.in'); reset(f);
readln(f,n);
for i:=1 to n do read(f,v[i]);
close(f);
end;

procedure rezolvare;
var i,j:integer;
begin

suma:=0;
fillchar(aparitie,sizeof(aparitie),0);
fillchar(media,sizeof(media),0);

for i:=1 to n-1 do
    begin
    inc(aparitie[v[i]]);
    for j:=i+1 to n do
        if (v[i]+v[j]) mod 2=0 then media[(v[i]+v[j]) div 2]:=media[(v[i]+v[j]) div 2]+1;
    end;

aparitie[v[n]]:=aparitie[v[n]]+1;

for i:=1 to n do
    if media[v[i]]<>0 then suma:=suma+media[v[i]]-aparitie[v[i]]+1;

end;

procedure afisare;
begin
assign(g,'medie.out'); rewrite(g);
writeln(g,suma);
close(g);
end;

begin
citire;
rezolvare;
afisare;
end.