var f,g:text;
    v:array[0..30]of double;
    a:array[0..30,0..30]of double;
    p:double;
    n,k,i,j:longint;
begin
assign(f,'dezastru.in');
assign(g,'dezastru.out');
reset(f);
rewrite(g);
read(f,n,k);
for i:=1 to n do read(f,v[i]);
for i:=0 to n do
  a[i,0]:=1;
for i:=1 to k do
  a[0,i]:=0;
for i:=1 to n do
  for j:=1 to k do
    a[i,j]:=a[i-1,j-1]*v[i]+a[i-1,j];
p:=1;
for i:=n-k+1 to n do p:=p*i;
p:=p/ 2;
write(g,a[n,k]/p:0:6);
close(f);
close(g);
end.