var f,g:text;
    st:array[1..30]of integer;
    a,b:array[1..30]of real;
    i,n,m,nr:longint;
    s:real;

function ok(k:longint):boolean;
begin
ok:=true;
for i:=1 to k-1 do
    if st[i]=st[k] then ok:=false;
end;

procedure afis;
begin
inc(nr);
b[nr]:=1;
for i:=1 to m do
    b[nr]:=b[nr]*a[st[i]];
end;

procedure back(k:longint);
var i:longint;
begin
if k>n then afis
       else for i:=1 to n do
                 begin
                 st[k]:=i;
                 if ok(k) then back(k+1);
                 end
end;

begin
assign(f,'dezastru.in');reset(f);
assign(g,'dezastru.out');rewrite(g);
readln(f,n,m);
for i:=1 to n do
    read(f,a[i]);
back(1);
s:=0;
for i:=1 to nr do
    s:=s+b[i]/nr;
writeln(g,s:8:6);
close(f);
close(g);
end.