program media;
var v,w:array[1..7000] of integer;
a:array[1..9000] of integer;
summax:longint;
f,g:text;
m,n,i,j,k:integer;
begin
 assign(f,'medie.in'); assign(g,'medie.out'); reset(f); rewrite(g);
 read(f,n);
 m:=n;
 j:=0;
 for i:=1 to m do
 begin
   read(f,a[i]);
   w[a[i]]:=w[a[i]]+1;
 end;
 summax:=0;
 for i:=1 to n-1 do
   for j:=i+1 to n do
     if ((a[i]+a[j]) mod 2=0) then v[(a[i]+a[j]) div 2]:=v[(a[i]+a[j]) div 2]+1;
 for i:=1 to n do
   summax:=summax+v[a[i]]-w[a[i]]+1;
 writeln(g,summax);
 close(f); close(g);
end.