object Main {
  class Solver(n: Int, s: String) {
    def nChar: (String, String) => Int = {
      def sub: (Array[Char], Array[Char], Char) => Int  = {
        (x, y, c) => c match {
          case 'z' => {
            if (x.contains(c) && y.contains(c)) 1
            else 0
          }
          case _ => {
            if (x.contains(c) && y.contains(c)) sub(x, y, (c+1).toChar) + 1
            else sub(x, y, (c+1).toChar)
          }
        }
      }
      (x, y) => sub(x.toCharArray, y.toCharArray, 'a')
    }

    def solve: Int = {
      val li = for (i <- 1 to n-1) yield nChar(s.slice(0, i), s.slice(i, n))
      li.max
    }
  }
  
  def main(args: Array[String]): Unit = {
    val n = io.StdIn.readLine().toInt
    val s = io.StdIn.readLine()
    val solver = new Solver(n, s)
    println(solver.solve)
  }
}

