#include<iostream>
#include<cmath>
#include<cstdio>
#include<cstring>
using namespace std;
char c[1050];
void FBI(int l,int r)
{
	int mid=(l+r)/2;
	if(l!=r)
	{
		FBI(l,mid);
		FBI(mid+1,r);
	}
	int x=0,y=0;
	for(int i=l;i<=r;i++)
	{
		if(c[i]=='0')
		{
			x++;
		}
		else
		{
			y++;
		}
		if(x&&y)
		{
			cout<<"F";
		}
		else if(x)
		{
			cout<<"B";
		}
		else
		{
			cout<<"I";
		}
	}
}
int main()
{
	int n;
	scanf("%d",n);
	scanf("%s",c+1);
	FBI(1,1<<n);
	return 0;
}