{dezastru preONI2007}
{$N+}

var a:array[1..25] of real;
    st:array[0..25] of byte;
    marc:array[1..25] of boolean;
    suma:extended;
    nr,n,k:byte;
    f,g:text;

procedure citire;
var i:byte;
begin
assign(f,'dezastru.in'); reset(f);
readln(f,n,k);
for i:=1 to n do read(f,a[i]);
close(f);
end;

procedure back(p:integer);
var i,j:byte;
begin
for i:=st[p-1]+1 to n do
    begin
    st[p]:=i;
    if p=k then
       begin
       inc(nr);
       for j:=1 to p-1 do
           suma:=suma+a[st[j]]*a[st[j+1]];
       end
    else back(p+1);
    end;
end;

procedure afisare;
var val:extended;
begin
assign(g,'dezastru.out'); rewrite(g);
val:=suma/nr;
writeln(g,val:2:6);
close(g);
end;

begin
suma:=0;
nr:=0;
citire;
back(1);
afisare;
end.