func main()
	var s: []char :: cui@input()
	var n: int :: ^s
	var cum: []int :: #[n + 1]int
	for i(0, n - 1)
		do cum[i + 1] :: cum[i] + (s[i] = 'A' ?(1, -1))
	end for
	var l: dict<int, int> :: #dict<int, int>
	var r: dict<int, int> :: #dict<int, int>
	for i(0, n)
		var v: int :: cum[i]
		if(!l.exist(v))
			do l.add(v, i)
		end if
		do r.add(v, i)
	end for
	
	var ans: int :: 0
	for i(-n, n)
		do ans :: lib@max(ans, r.get(i, &) - l.get(i, &))
	end for
	do cui@print("\{ans}\n")
end func