{$N+}
var v:array[1..25] of extended;
a:array[1..25,0..25] of extended;
n,k,i,j,up,down:longint;
f,g:text;
begin
assign(f,'dezastru.in');
assign(g,'dezastru.out');
reset(f);rewrite(g);
readln(f,n,k);
for i:=1 to n do
    begin
    read(f,v[i]);
    a[i,0]:=1;
    end;
a[1,1]:=v[1];
for i:=2 to n do
    for j:=1 to i do
        a[i,j]:=a[i-1,j]+a[i-1,j-1]*v[i];
up:=1;down:=1;
for i:=k+1 to n do
    up:=up*i;
for i:=1 to n-k do
    down:=down*i;
write(g,a[n,k]/(up div down):5:6);
close(f);close(g);
end.