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