import java.util.Scanner;

public class _2 {
    static int n;
    static String s;

    static class treeNode{
        char c;
        treeNode left,right;
    }
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        n=sc.nextInt();
        s=sc.next();
        treeNode root=new treeNode();
        if(s.contains("0")&&s.contains("1"))root.c='F';
        else if(s.contains("0"))root.c='B';
        else root.c='I';
        buildtree(s,root);
        preorder(root);
    }

    static void buildtree(String s,treeNode root){
        if(s.length()==1)return;
        String s1=s.substring(0,s.length()/2);
        String s2=s.substring(s.length()/2);
        if(s1.contains("0")&&s1.contains("1")){
            root.left=new treeNode();
            root.left.c='F';
            buildtree(s1,root.left);
        }
        else if(s1.contains("0")){
            root.left=new treeNode();
            root.left.c='B';
            buildtree(s1,root.left);
        }
        else if(s1.contains("1")){
            root.left=new treeNode();
            root.left.c='I';
            buildtree(s1,root.left);
        }
        if(s2.contains("0")&&s2.contains("1")){
            root.right=new treeNode();
            root.right.c='F';
            buildtree(s2,root.right);
        }
        else if(s2.contains("0")){
            root.right=new treeNode();
            root.right.c='B';
            buildtree(s2,root.right);
        }
        else if(s2.contains("1")){
            root.right=new treeNode();
            root.right.c='I';
            buildtree(s2,root.right);
        }
    }

    static void preorder(treeNode root){
        if(root==null)return;
        preorder(root.left);
        preorder(root.right);
        System.out.print(root.c);
    }
}