#include<bits/stdc++.h>
using namespace std;
int dfs(string s) {
	if ((int)s.length() == 1) {
		if (s[0] == '0') {
			putchar('B');
			return 0;
		} else {
			putchar('I');
			return 1;
		}
	}
	string a, b;
	for (int i = 1; i < (int)s.length(); i++) {
		if (i < (int)s.length() / 2) {
			a += s[i];
		} else b += s[i];
	}
	int ansa = dfs(a), ansb = dfs(b);
	if (ansa == 0 && ansb == 0) {
		putchar('B');
		return 0;
	} else if (ansa == 1 && ansb == 1) {
		putchar('I');
		return 1;
	} else {
		putchar('F');
		return 2;
	}
}
int main() {
	int n;
	scanf("%d", &n);
	string s;
	cin >> s;
	dfs(s);
	return 0;
}