program medieeee; 
type vec=array[0..9000] of integer;
var a,b:vec;
n,i,j,k,s,e,max:integer;
nr,p:longint;
f,g:text;
begin
assign(f,'medie.in');assign(g,'medie.out');reset(f);rewrite(G);
readln(f,n); max:=0;
for i:=1 to n do begin
readln(f,a[i]);b[a[i]]:=b[a[i]]+1;if max<a[i] then max:=a[i];end;
k:=0;
for i:=1 to max do
if b[i]<>0 then begin k:=k+1;a[k]:=i;end;
n:=k;
for i:=1 to n do begin
if b[a[i]]>2 then begin
p:=b[a[i]]*(b[a[i]]-1) div 2;
p:=p*(b[a[i]]-2);nr:=nr+p;end;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];
e:=e*b[a[i]]*b[a[j]];
nr:=nr+e;
end;
end;
write(g,nr);
close(f);close(g);end.