S = gets.chomp.chars
A = [sum = 0]+S.map{|c|sum += c == ?A ? 1 : 0}
B = [sum = 0]+S.map{|c|sum += c == ?B ? 1 : 0}

ans = 0
l = 0
r = 0
while r < A.size
  a = A[r] - A[l]
  b = B[r] - B[l]
  ra = A[-1] - A[r]
  rb = B[-1] - B[r]
  if rb < a - b || ra < b - a
    l += 1
  else
    r += 1
  end
  if a == b
    ans = a*2 if a*2 > ans
  end
end
p ans