#include<bits/stdc++.h>
using namespace std;
int k, n;
char tree[2000];
string s;
void make(int i)
{
for (; i > 0; --i)
{
if (tree[i * 2] == 'I' && tree[i * 2 + 1] == 'I') tree[i] = 'I';
else if (tree[i * 2] == 'B' && s[i * 2 + 1] == 'B') tree[i] = 'B';
else tree[i] = 'F';
}
}
char judge(int x)
{
if (s[x] == '0') return 'B';
return 'I';
}
void print(int i)
{
if (i > n) return;
print(i * 2);
print(i * 2 + 1);
cout << tree[i];
}
int main()
{
cin >> k;
getchar();
cin >> s;
n = (int)pow(2, k + 1) - 1;
int len = (int)pow(2, k);
for (int i = 1; i <= n; i += 2)
{
tree[(k + i / 2 + 1) * 2] = judge(i - 1);
tree[(k + i / 2 + 1) * 2 + 1] = judge(i);
}
make(n / 2);
print(1);
}