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