class node:
def init(self, mark):
self.mark = mark
self.left = None
self.right = None

def str_type(string):
if not '0' in string:
return 'I'
if not '1' in string:
return 'B'
return 'F'

def build_tree(string):
root = node(str_type(string))
if len(string) == 1:
return root
root.left = build_tree(string[:len(string)//2])
root.right = build_tree(string[len(string)//2:])
return root

def generate_post_seq(tree):
if not tree:
return ''
return generate_post_seq(tree.left) + generate_post_seq(tree.right) + tree.mark

_ = input()
string = input().strip()
print(generate_post_seq(build_tree(string)))