var n,k,i,j:longint;
    a:array[0..30,0..30] of real;
    b:array[0..30] of real;

  function comb(n,k:byte):real;
   var i:longint;
    begin
     comb:=1;
      for i:=1 to n do
       begin
        comb:=comb*i;
        if i<=n-k then comb:=comb/i;
        if i<=k then comb:=comb/i;
       end;
    end;

begin
 assign(input,'dezastru.in');
 assign(output,'dezastru.out');
 reset(input);
 rewrite(output);

   readln(n,k);
    for i:=1 to n do
     begin
      read(b[i]);
      a[i][0]:=1;
     end;

    a[0][0]:=1;
    for i:=1 to k do
     for j:=i to n do
      a[j][i]:=a[j-1][i]+a[j-1][i-1]*b[j];

   writeln(a[n][k]/comb(n,k):0:6);

 close(input);
 close(output);
{Totusi este trist in lume}
end.