program back;
type vect=array[1..200] of integer;
type vecta=array[1..200] of double;
var v:vect;
    f,g:text;
    n,k:longint;
    i,fact,fact2:longint;
    prod,sum:double;
    u:vecta;
{}
procedure cool(poz,nod:integer);
var i,q:integer;
begin
if poz=k+1 then begin
        prod:=1;
        for q:=1 to k do
         prod:=prod*u[v[q]];
         sum:=sum+prod*fact;



end
else
for i:=nod+1 to n do begin
             v[poz]:=i;
             cool(poz+1,i);
             end;
end;
{}
begin
assign(f,'dezastru.in'); reset(f);
assign(g,'dezastru.out'); rewrite(g);
read(f,n,k);
for i:=1 to n do read(f,u[i]);
fact:=1;
for i:=1 to k do
    fact:=fact*i;
fact2:=fact;
for i:=k+1 to n do
    fact2:=fact2*i;
sum:=0;
cool(1,0);
sum:=sum/fact2;
writeln(g,sum:0:6);
close(f); close(g);
end.