def FBI(s):
    if '1' in s:
        if '0' in s:
            return 'F'
        else:
            return 'I'
    else:
        return 'B'

class Node:
    def __init__(self, s):
        self.s = s
        self.data = FBI(s)
        self.left = None
        self.right = None
        if len(s) > 1:
            self.left = Node(s[:len(s)//2])
            self.right = Node(s[len(s)//2:])

def order(node):
    if node.left:
        order(node.left)
    if node.right:
        order(node.right)
    print(node.data,end='')

N=input()
s=input()
order(Node(s))