#include<bits/stdc++.h>
using namespace std;
int n;
char x;
char dfs(int step){
	if(step==n){
		cin>>x;
		if(x=='1'){
			cout<<'I';
			return 'I';
		}else{
			cout<<'B';
			return 'B';
		}
	}else{
		char l=dfs(step+1);
		char r=dfs(step+1);
		if(l=='B'&&r=='B'){
			cout<<'B';
			return 'B';
		}
		else if(l=='I'&&r=='I'){
			cout<<'I';
			return 'I';
		}
		else{
			cout<<'F';
			return 'F';
		}
	}
}
int main(){
    cin>>n;
    dfs(0);
	return 0;
}
/**************************************************************
	Problem: 2246
	User: fuhongyi
	Language: C++
	Result: Accepted
	Time:118 ms
	Memory:2072 kb
****************************************************************/