program main
    implicit none
    integer::i,j,k,x,y,r,ans
    integer,parameter::MAX_N=200000
    integer::n,ma(-MAX_N:MAX_N),mi(-MAX_N:MAX_N)
    character(MAX_N)::s
    read*,s
    n=len_trim(s)
    ma(0)=0
    mi(0)=0
    do i=-n,n
        ma(i)=-1
        mi(i)=n+1
    end do
    r=0
    do i=1,n
        if(s(i:i)=='A')then
            r=r+1
        else
            r=r-1
        end if
        ma(r)=max(ma(r),i)
        mi(r)=min(mi(r),i)
    end do
    ans=0
    do i=-n,n
        ans=max(ans,ma(i)-mi(i))
    end do
    print*,ans
end program