var f,g:text;
x:array[1..9000] of integer;
a,b:array[1..7000] of integer;
max,n,i,j,k,l:longint;
begin
assign(f,'medii.in');
assign(g,'medii.out');
reset(F);
rewrite(G);
readln(f,n);
max:=0;
for i:=1 to n do
    begin
    readln(f,x[i]);
    a[x[i]]:=a[x[i]]+1;
    if x[i]>max then max:=x[i];
    end;
for i:=1 to n do
    for j:=i+1 to n do
        if( x[i]+x[j] )mod 2=0 then
           inc(b[(x[i]+x[j])div 2]);
for i:=1 to max do
    if (b[i]>0)and(a[i]>0) then
          k:=k+a[i]*(b[i]-a[i]+1);
writeln(g,k);
close(F);
close(G);
end.