program dezastru;
var n,k: 0..25;
v: array[1..25] of real;
a: array[1..25] of integer;
c,i: integer;
p: real;
f: text;
procedure calc;
var x: real;
i: integer;
begin
     x:=1;
     for i:=1 to k do
         x:=x*v[a[i]];
     p:=p+x;
end;
procedure bkt(p: longint);
var i: longint;
begin
     if p=k+1 then
     begin
     inc(c);
     calc;
     end
     else
         for i:=a[p-1]+1 to n-k+p do
         begin
              a[p]:=i;
              bkt(p+1);
         end;
end;
begin
     assign(f,'dezastru.in');
     reset(f);
     readln(f,n,k);
     for i:=1 to n do
         read(f,v[i]);
     close(f);
     bkt(1);
     assign(f,'dezastru.out');
     rewrite(f);
     write(f,p/c:6:6);
     close(f);
end.