var a:array[1..25] of double;  b:array[0..25,0..25] of double;    n,k:longint;  
 procedure citesc;  
  var i:longint;  
      f:text;  
  begin  
   assign(f,'dezastru.in'); reset(f);  read(f,n,k);  
   for i:=1 to n do  read(f,a[i]);  
   for i:=0 to n do begin  
    b[0,i]:=0;  
    b[i,0]:=1;  
   end;  
   close(f);  
  end;  
 function aflu:double;  
  var i,j:longint;  q:double;  
  begin  
   q:=1;  
    for i:=k+1 to n do       q:=q*i;  
    for i:=1 to n-k do       q:=q/i;  
   aflu:=q;  
  end;  
 procedure rezolvare;  
  var i,j:longint;     g:text;   t,q:double;  
  begin  
   for i:=1 to n do  
    for j:=1 to k do    b[i,j]:=b[i-1,j]+b[i-1,j-1]*a[i];  
   q:=aflu;  
   assign(g,'dezastru.out'); rewrite(g);  
   t:=(b[n,k])/q;     writeln(g,t:0:6);  
   close(g);  
  end;  
 begin  
  citesc;  
  rezolvare;  
 end.