program medieeee;
type vec=array[1..9000] of integer;
var a,b:vec;
    n,i,j,k,s,e:integer;
    nr:longint;
    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]);b[a[i]]:=b[a[i]]+1;;end;
for i:=1 to n-1 do
for j:=i+1 to n do begin
s:=(a[i]+a[j]) div 2;
if a[i]+a[j]=s*2 then begin
e:=b[s];
if a[i]=s then e:=e-1;
if a[j]=s then e:=e-1;
nr:=nr+e;
end;
end;
write(g,nr);
close(f);close(g);end.