proc getchar_unlocked():char {. importc:"getchar_unlocked",header: "<stdio.h>" .}
template `max=`*(x,y) = x = max(x,y)
template `min=`*(x,y) = x = min(x,y)
var X : array[-200010..200010,int]
var xi = 0
var ans = 0
for i in 1..<4e6.int:
  let k = getchar_unlocked()
  if k < 'A' : break
  if k == 'A' :
    if X[xi] == 0 : X[xi] = i
    ans .max= i - X[xi]
    xi += 1
  else:
    if X[xi] == 0 : X[xi] = i
    ans .max= i - X[xi]
    xi -= 1
echo ans