var f,g:text;
    j,i,l,n,k:integer;
    v,p:array[1..20] of real;
    rezultat:real;
begin
assign(f,'dezastru.in');reset(f);
assign(g,'dezastru.out');rewrite(g);
read(f,n,k);
readln(f);
for i:=1 to n do
    read(f,v[i]);
repeat
inc(j);
p[j]:=1;
for i:=1+l to k+l do
    p[j]:=p[j]*v[i];
inc(l);
until k+l-1>=n;
inc(j);
p[j]:=v[1]*v[n];
for i:=1 to j do
rezultat:=rezultat+((p[i]*2)/6);
writeln(g,rezultat:2:6);
close(f);close(g);
end.