String clean(String input) {
  
  //check for brackets
  int opened = 0; int havetoclose = 0; int havetoopen = 0;
  
  for(int j=0; j<input.length(); j++) {
    if(input.charAt(j) == '[') {
      opened++;
    } else if(input.charAt(j) == ']') {
      if(opened>0) opened--; else havetoopen++;
    }
  }
  while(opened>0) {
    opened--; havetoclose++;
  }
  while(havetoclose>0) {
    havetoclose--; input+=']';
  }
  while(havetoopen>0) {
    havetoopen--; input='['+input;
  }
  for(int j=1; j<input.length()-1; j++) {
    if(input.charAt(j)=='['&& input.charAt(j+1)==']') {
      input = input.substring(0,j)+input.substring(j+2);
    }
  }
  
  
  //check for uselessness ... forward and back movements
  for(int j=1; j<input.length()-1; j++) {
    if( (input.charAt(j)=='b'&&input.charAt(j+1)=='r') ||  (input.charAt(j)=='r'&&input.charAt(j+1)=='b')) {
      input = input.substring(0,j)+input.substring(j+2);
    }
  }
  return input;
 }