const

 m = 100;

var

 n, i, k: integer;

 s: real;

 Y: array[1..m] of real;

begin

 read(n);

 s := 0;

 k := 0;

 for i := 1 to n do

 begin

   read(Y[i]);

   if (i mod 2 = 0) and (Y[i] > 0) then

   begin

     s := s + Y[i];

     k := k + 1;

   end;

 end;

 s := s / k;

 write(s);

end.