#include <iostream>
#include <cmath>
using namespace std;
void FBI(string s, int l, int r){
    if(l == r){
        if(s[l] == '0') cout << 'B';
        else    cout <<  'I';
        return ;
    }
    FBI(s, l, l + r >> 1);
    FBI(s, (l + r >> 1) + 1, r);
    bool B, I;
    B = I = false;
    for(int i = l; i <= r; i ++ ){
        if(s[i] == '0')     B = true;
        else    I = true;
    }
    if(B && I)  cout << 'F';
    else if(B)  cout << 'B';
    else    cout << 'I';
}
int main(){
    int n;  
    string s;
    cin >> n >> s;
    FBI(s, 0, int(pow(2, n)) - 1);
    return 0;
}