program resistance;
var N_sides, n_resistors: integer;
var min_resistance, answer, temp: real;
begin
  min_resistance := 9999999;
  answer := 0;
  read(N_sides, n_resistors);
  for var i := 0 to n_resistors - 1 do
  begin
    temp := readReal();
    if temp < min_resistance then
      min_resistance := temp;
  end;
  if (N_sides = 4) or (N_sides = 8) or (N_sides = 20) then
  begin
    answer := min_resistance / 2.0;
  end
  else
    begin
    if N_sides = 6 then
    begin
      answer := min_resistance / 6.0 * 5.0;
    end
    else
      begin
      answer := min_resistance / 6.0 * 7.0;
      end
    end;
  answer := Round(answer, 5);
  WritelnFormat('{0:f5}', answer);
end.