{$N+}  
var v:array[1..25] of 1..25;  
u:array[1..25] of extended;  
n,k,i,fact,j:integer;  
s,p:extended;  
f,g:text;  
ok,l:boolean;  
begin  
assign(f,'dezastru.in');  
assign(g,'dezastru.out');  
reset(f);rewrite(g);  
readln(f,n,k);  
for i:=1 to n do  
    read(f,u[i]);  
fact:=1;  
for i:=k+1 to n do  
    fact:=fact*i;
for i:=2 to n-k do
    fact:=fact*i;
for i:=1 to k do  
    v[i]:=i;  
repeat  
      if ok=true then  
         l:=true;  
      p:=1;  
      for i:=1 to k do  
          p:=p*u[v[i]];  
      s:=s+p;  
      if n=k then  
         break;  
      i:=k;  
      while k-i>n-v[i]-1 do  
            i:=i-1;  
      v[i]:=v[i]+1;  
      for j:=i+1 to k do  
          v[j]:=v[i]+j-i;  
      ok:=true;  
      if v[k]<>n then  
         ok:=false  
      else  
          for i:=k downto 2 do  
              if v[i]<>v[i-1]+1 then  
                 begin  
                 ok:=false;  
                 break;  
                 end;  
until l=true;  
write(g,s/fact:5:6);  
close(f);close(g);  
end.