const maxn = 25;

var f:text;
    n,k,i,j:longint;
    sol,r:real;
    a:array[1..maxn]of real;
    c:array[1..maxn,1..maxn]of real;

procedure readdata;
begin
   assign(f,'dezastru.in');
   reset(f);
   readln(f,n,k);
   for i:=1 to n do
   begin
      read(f,a[i]);
   end;
   close(f);
end;

procedure solve;
begin
   c[1,1]:=a[1];
   for i:=2 to n do c[i,1]:=c[i-1,1]+a[i];
   for i:=2 to n do
   begin
      for j:=2 to k do
      begin
         c[i,j]:=c[i-1,j]+c[i-1,j-1]*a[i]
      end;
   end;
end;

procedure writedata;
begin
   assign(f,'dezastru.out');
   rewrite(f);
   sol:=c[n,k];
   r:=1;
   for i:=k+1 to n do
   begin
      r:=r*i/(i-k);
   end;
   writeln(f,sol/r:0:6);
   close(f);
end;

begin
   readdata;
   solve;
   writedata;
end.