s=gets.chomp
sum=[0]
s.each_byte{|b|sum << sum.last+(b==?A.ord ? 1 : -1)}

h={}
sum.each_with_index{|v,i|
  if h[v]
    h[v][1]=i
  else
    h[v]=[i,-1]
  end
}

max=0
h.each_value{|l,r|
  next if r==-1
  d=r-l
  max=d if d>max
}

puts max