program dezastru;
type vect=array[0..100] of real;
var u,a,b:vect;
    f,g:text;
    n,k,i,perm,j,comb:longint;
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]);
{first step}
b[1]:=u[1]; perm:=1;
for j:=2 to k do
 b[j]:=b[j-1]*perm;
{later}
for i:=2 to n do begin
   a:=b;    perm:=perm*i;
   b[i]:=u[i];
   for j:=2 to k do
      b[j]:=a[j]+b[j-1]*perm;
   end;
a:=b;
comb:=1;
for i:=n downto n-k+1 do
          comb:=comb*i;
for i:=1 to k do
         comb:=comb div i;
b[n]:=b[n]/comb;
writeln(g,b[n]:0:6);
close(f); close(g);
end.