package main

func main() {
	println(getAns("ABAAABBBA"))
	println(getAns("ABAAAABBBA"))
	println(getAns("ABAAAAABBBA"))
	println(getAns("AAAAAA"))
	println(getAns("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"))
}

func getAns(S string) int {
	length := len(S)
	sum := 0
	sumArr := make([]int, length)
	posMap := make(map[int]int)
	for i := 0; i < length; i++ {
		if S[i] == 'A' {
			sum++
		} else {
			sum--
		}
		sumArr[i] = sum
		posMap[sum] = i
	}
	if sum == 0 {
		return length
	} else if sum == length || sum == -length {
		return 0
	}
	maxLen := 0
	for i := 0; i < length; i++ {
		if sumArr[i] != 0 && maxLen < posMap[sumArr[i]]-i {
			maxLen = posMap[sumArr[i]] - i
		}
	}
	if posMap[0] != posMap[len(posMap)-1] && posMap[0]+1 > maxLen {
		maxLen = posMap[0] + 1
	}
	return maxLen
}