void main(){
  import std.stdio, std.string, std.conv, std.algorithm;

  auto s=readln.chomp.to!(char[]);

  int n=s.length.to!(int);
  bool can(int len){
    len+=(len&1);
    if(len>n) return false;
    int na=0, nb=0;
    foreach(i; 0..n){
      if(i<len){
        (s[i]=='A'?na:nb)+=1;
      }else{
        (s[i-len]=='A'?na:nb)-=1;
        (s[i]=='A'?na:nb)+=1;
      }
      if(na==nb && na+nb>=len) return true;
    }
    return false;
  }

  int ok=-1, ng=n+1;
  while(ng-ok>1){
    auto m=(ng+ok)/2;
    // writeln(ok, " ", m, " ", ng);
    (can(m)?ok:ng)=m;
  }
  writeln(ok);
}

void rd(T...)(ref T x){
  import std.stdio, std.string, std.conv;
  auto l=readln.split;
  assert(l.length==x.length);
  foreach(i, ref e; x) e=l[i].to!(typeof(e));
}