var v:array[1..9000] of integer;
ap1,ap2:array[1..18000] of integer;
n,i,j,k:longint;
nr:int64;
f:text;
begin
assign(f,'medie.in');reset(f);
read(f,n);
for i:=1 to n do begin
read(f,v[i]);
inc(ap1[v[i]]);
inc(ap2[2*v[i]]);
end;
nr:=0;
for i:=1 to n do begin
    for j:=i+1 to n-1 do
    nr:=nr+ap2[v[i]+v[j]];
    end;
for i:=1 to n do
nr:=nr-ap1[i]+1;
assign(f,'medie.out');rewrite(f);
write(f,nr);
close(f);
end.