var v:array[1..30]of real;
    n,k,i,j,g:integer;
    st:array[0..30]of integer;
    s:real;

procedure back(g:integer);
var i:integer;
    t:real;
begin
if g=k then
        begin
        t:=1;
        for i:=1 to k do
                t:=t*v[st[i]]*i;
        s:=s+t;
        end
else
        begin
        for i:=st[g]+1 to n-(k-(g+1)) do
                begin
                st[g+1]:=i;
                back(g+1);
                end;
        end;
end;

begin
assign(input,'dezastru.in');reset(input);
assign(output,'dezastru.out');rewrite(output);
readln(n,k);
for i:=1 to n do
        read(v[i]);
back(0);
for i:=n-k+1 to n do
        s:=s/i;
writeln(s:0:6);
close(input);close(output);
end.