program usor;
var i,n,k,nr,j,x:longint;
    a:array[1..25] of real;
    s:real;
procedure comb;
var j:integer;
begin
     while x<k do begin
           inc(x);
     for j:=i+1 to n do begin
        inc(nr);
        s:=s+a[i]*a[j];
        end;
    for j:=i-1 downto 1 do begin
        inc(nr);
        s:=s+a[i]*a[j];
        end;
    comb;
    end;
end;
begin
assign(input,'dezastru.in');
reset(input);
read(input,n,k);
for i:=1 to n do
    read(input,a[i]);
close(input);
nr:=0;s:=0;
for i:=1 to n do begin
    x:=1;
    comb;
    end;
s:=s/nr;
assign(output,'dezastru.out');
rewrite(output);
write(output,s:6:6);
close(output);
end.
end.