#include<bits/stdc++.h>
using namespace std;
string s;
int n;
void aaa(string x){
	string x1,x2;
	int flag;
	for(int i=0;i<x.size();i++){
		if(i<x.size()/2)x1+=x[i];
		else x2+=x[i];
		if(!flag){
			if(x[i]-'0'==0)flag=1;
			if(x[i]-'0'==1)flag=2;
		}else if(flag==1&&x[i]-'0'==0)flag=3;
		else if(flag==0&&x[i]-'0'==1)flag=3;
	}
	aaa(x1);
	aaa(x2);
	if(flag==1)printf("B");
	if(flag==2)printf("I");
	if(flag==3)printf("F");
	return;
}
int main(){
//	freopen("","r",stdin);
//	freopen("","w",stdout);
		
	scanf("%d",&n);
	cin>>s;
	aaa(s);
	return 0;
}