var f,g:Text;
    b:array[0..100]of longint;
    a:array[0..100]of double;
    n,k,i,j:longint;
    s,c,p:double;

begin
assign(f,'dezastru.in');
assign(G,'dezastru.out');
reset(f);
rewrite(G);
read(f,n,k);
for i:=1 to n do
  begin
    read(f,a[i]);
    b[i]:=i;
  end;
c:=1;
for i:=k+1 to n do c:=c*i;
for i:=2 to n-k do c:=c/i;
c:=c*2;
while true do
  begin
    p:=1;
    for i:=1 to k do p:=p*a[b[i]];
    s:=s+(p/C);
    i:=k;
    while (b[i]>=n-k+i)do dec(i);
    if (i=0)then break;
    inc(b[i]);
    for j:=i+1 to k do
      b[j]:=b[j-1]+1;
  end;
write(g,s*2:6:6);
close(f);
close(g);
end.