let inpt = readLine()!.map{ $0 }
var aCount = [0]
var bCount = [0]
if inpt.first! == "A" {
    aCount[0] = 1
} else {
    bCount[0] = 1
}

for i in(1..<inpt.count) {
    aCount.append(aCount.last!)
    bCount.append(bCount.last!)
    if inpt[i] == "A" {
        aCount[i] = aCount[i] + 1
    } else {
        bCount[i] = bCount[i] + 1
    }
}

var ans = 0
for i in (0..<inpt.count) {
    var j = inpt.count - 1
    if (j-i) % 2 == 0 {
        j -= 1
    }
    while j > i {
        let len = j - i + 1
        if(len <= ans) {
            break
        }
        var aCnt = aCount[j]
        var bCnt = bCount[j]
        if(i>0) {
            aCnt -= aCount[i-1]
            bCnt -= bCount[i-1]
        }
        if(aCnt < 0 || bCnt < 0) {
            break
        }
        if aCnt == bCnt {
            ans = max(ans, len)
            break
        }
        j -= 2
    }
}
print(ans)