import java.util.Scanner

object Main {
  def max(a: Int, b: Int): Int = {
    if (a < b) b else a
  }

  def min(a: Int, b: Int): Int = {
    if (a < b) a else b
  }

  def solve(sc: => Scanner): Unit = {
    val s = sc.next
    val (_, _, maxcnt, _) = (s + ' ').foldLeft((0, 0, 0, ' '))((acc, c) =>
      (acc, c) match {
        case ((acnt, bcnt, maxcnt, prev), c) if prev != c =>
          (if (c == 'A') 1 else acnt, if (c == 'B') 1 else bcnt, max(maxcnt, min(acnt, bcnt)), c)
        case ((acnt, bcnt, maxcnt, prev), 'A') => (acnt + 1, bcnt, maxcnt, 'A')
        case ((acnt, bcnt, maxcnt, prev), 'B') => (acnt, bcnt + 1, maxcnt, 'B')
    })
    println(maxcnt*2)
  }

  def main(args: Array[String]): Unit = {
    val sc: Scanner = new Scanner(System.in)
    solve(sc)
  }
}