package main

import "fmt"

const MAX_LEN = 200000

func max(a int, b int) int {
	if a >= b {
		return a
	} else {
		return b
	}
}

func main() {
	var S string
	fmt.Scan(&S)

	scores := [MAX_LEN]int{}
	offset := map[rune]int{'A': 1, 'B': -1}

	for i, c := range S {
		scores[i] += offset[c]
		if i > 0 {
			scores[i] += scores[i-1]
		}
	}

	table := map[int]int{}

	ans := 0
	for l := len(S) - 1; l >= 0; l-- {
		table[scores[l]] = max(table[scores[l]], l)

		r, ok := table[scores[l] - offset[rune(S[l])]]

		if ok {
			ans = max(ans, r - l + 1)
		}
	}

	fmt.Println(ans)
}