#include<bits/stdc++.h>
using namespace std;
char a[1040];
int n;
char FBITREE(int l,int r){
if (l == r) if (a[l] == '1') return 'I'; else return 'B';
char fl = FBITREE(l,(l+r)/2); cout<<fl;
char fr = FBITREE((l+r)/2+1,r); cout<<fr;
if (fl == fr) return fl; else return 'F';
}
int main(){
cin>>n>>a;
int al = strlen(a);
cout<<FBITREE(0,al-1)<<endl;
}

AC了没？