var v : array[0..26] of double;
    a : array[0..26,0..26] of double;
    n,i,k,j : longint;
    f,g : text;
    p,q : double;
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:=0 to n do
      a[0,i]:=0;
    for i:=0 to n do
      a[i,0]:=1;

    for i:=1 to n do
      for j:=1 to k do
        a[i,j]:=a[i-1,j]+a[i-1,j-1]*v[i];
  p:=1;
    for i:=k+1 to n do
      p:=p*i;
    for i:=1 to n-k do
      p:=p/i;
  q:=a[n,k]/p;
  writeln(g,q:0:6);
  close(g);
end.