#include <vector>
#include <iostream>
using namespace std;
int curr;
int len[1000001];
char last[1000001];
int letter_anc[1000001][20];
void Init()
{
curr = 0;
len[0] = -1;
last[0] = '?';
for(int e = 0; e < 20; e++)
letter_anc[0][e] = 0;
}
void TypeLetter(char L)
{
curr++;
len[curr] = len[curr-1] + 1;
last[curr] = L;
letter_anc[curr][0] = curr-1;
for(int e = 1; e < 20; e++)
{
letter_anc[curr][e] = letter_anc[ letter_anc[curr][e-1] ][e-1];
}
// cerr << curr << '\n';
}
void UndoCommands(int U)
{
int prev = curr - U;
// cerr << "making copy of " << prev << '\n';
curr++;
len[curr] = len[prev];
last[curr] = last[prev];
letter_anc[curr][0] = letter_anc[prev][0];
for(int e = 1; e < 20; e++)
{
letter_anc[curr][e] = letter_anc[ letter_anc[curr][e-1] ][e-1];
}
// cerr << curr << '\n';
}
char GetLetter(int P)
{
int pos = curr;
for(int e = 19; e >= 0; e--)
if(len[ letter_anc[pos][e] ] >= P)
pos = letter_anc[pos][e];
return last[pos];
}
