var a,c:array[0..25]of real;
    d:array[0..25]of integer;
    b:array[1..5000000]of real;
    n,k,i,j,z,x,l:longint;
    r:real;

procedure back(i:longint);
var j:longint;
begin
     if i>k then
     begin
          z:=z+1;
          b[z]:=1;
          for j:=1 to k do b[z]:=b[z]*c[j];
          exit;
     end;

     for j:=d[i-1] to n do
     begin
          if a[j]=c[x] then continue;
          x:=x+1;
          d[x]:=j;
          c[x]:=a[j];
          back(i+1);
          c[x]:=0;
          d[x]:=0;
          x:=x-1;
     end;

end;


begin
     assign(input,'DEZASTRU.IN');
     reset(input);
     assign(output,'DEZASTRU.OUT');
     rewrite(output);
     readln(n,k);
     for i:=1 to n do read(a[i]);
     d[0]:=1;
     back(1);
     for i:=1 to z do
     begin
          r:=r+2*b[i];
     end;
     r:=r/(z*2);
     writeln(r:0:6);
close(output);
end.