uses math;
var a:array[0..trunc(1e5)+7] of longint;
    i,n:longint;
    ans,res:real;
procedure sort(l,r:longint);
var i,j,x,tmp:longint;
begin
i := l; j := r; x := a[(i+j) div 2];
 repeat
   while a[i] < x do inc(i);
   while a[j] > x do dec(j);
   if (i <= j) then
     begin
        tmp := a[i];
        a[i] := a[j];
        a[j] := tmp;
        inc(i);dec(j);
     end;
 until (i > j);
if (l<j) then sort(l,j);
if (i<r) then sort(i,r);
end;
begin
assign(input,'TGLN.inp');reset(input);
assign(output,'TGLN.out');rewrite(output);
readln(n);
for i := 1 to n do read(a[i]);
sort(1,n);
res := -maxlongint;
for i := 3 to n do
 if (a[i-2] + a[i-1] > a[i]) then
  begin
    ans := (a[i-2] + a[i-1] + a[i]) / 2;
    res := max(res,sqrt(ans*(ans-a[i-2])*(ans-a[i-1])*(ans-a[i])));
  end;
if res = -maxlongint then writeln(-1) else writeln(res:0:3);
close(input);close(output);
end.