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#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';
