program medieeee;
type vec=array[1..10000] of integer;
var a,b:vec;
    n,i,j,k,s,e:integer;
    nr:real;
    f,g:text;
begin
assign(f,'medie.in');assign(g,'medie.out');reset(f);rewrite(G);
readln(f,n);
for i:=1 to n do begin
readln(f,a[i]);inc(b[a[i]]);end;
for i:=1 to n-1 do
for j:=i+1 to n do begin
s:=a[i]+a[j];
if s=(s div 2)*2 then begin
s:=s div 2;
e:=b[s];
if a[i]=s then e:=e-1;
if a[i]=s then e:=e-1;
nr:=nr+e;
end;
end;
write(g,nr:0:0);
close(f);close(g);end.