#include <bits/stdc++.h>

using namespace std;

//声明FBI函数
char FBI(string s);

int main() {
    int n;
    cin >> n;
    string s;
    cin >> s;
    cout << FBI(s);
    return 0;
}

//FBI函数（递归函数）的实现
char FBI(string s) {
    if (s.length() > 1) {
        //输出左
        cout << FBI(s.substr(0, s.length() / 2));
        //输出右
        cout << FBI(s.substr(s.length() / 2, s.length() / 2));
    }
    //  http://c.biancheng.net/view/400.html
    if (s == string(s.length(), '0')) return 'B'; //通过string的构造函数，构建s.length()长度的0组成的字符串
    if (s == string(s.length(), '1')) return 'I';
    //混搭的输出F
    return 'F';
}