puts (1..read_line.to_i).map {
  read_line.chars.map(&.-('a'))
}.select { |s|
  s.each_cons(2).all? { |(i, j)| i <= j }
}.sort_by { |s|
  {s.last, s.first}
}.each_with_object([0] * 26) { |s, dp|
  dp[s.last] = {dp[s.last], dp[..s.first].max + s.size}.max
}.max