var n,k,i,j:longint;
fact:real;
a:array[0..26,0..26] of real;
v:array[1..26] of real;
f,g:text;
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,v[i]);
for i:=1 to n do
a[i,j]:=a[i-1,j]+v[i];
for i:=2 to k do
for j:=1 to n do
a[j,i]:=a[j-i,i]+a[j-1,i-1]*v[i];
fact:=1;
for i:=1 to k do
fact:=fact*(n-i+1)/i;
write(g,a[n,k]/fact:0:6);
close(f);
close(g);
end.