#include<iostream>
using namespace std;

string s;
char hasType(int left,int right) {
	bool hasZero=false, hasOne = false;
	for (int i = left; i <= right; i++) {
		if (s[i] == '0') hasZero = true;
		else if (s[i] == '1')hasOne = true;
		if (hasZero && hasOne) return 'F';
	}
	if (hasZero)return 'B';
	return 'I';
}
void maketree(int left, int right) {
	if (left != right) {
		int mid = (left + right) / 2;
		maketree(left, mid);
		maketree(mid + 1, right);
	}
	cout << hasType(left, right);
	
}
int main() {
	int n;
	cin >> n;
	cin >> s;
	maketree(0,s.size()-1);

	return 0;
}