Canadian Computing Competition: 2025 Stage 1, Senior #2

Cyrene, the captain of the Cryptogram Cracking Club (CCC), came across a concerningly long cipher. Conveniently, this cipher is composed of lower-case characters (a-z). Comfortingly, the cipher is composed of a pattern that repeats infinitely.

Cyrene wishes to locate the $c$-th character of the cipher. To make your job easier, the CCC members have extracted the repeated pattern and compressed it using the Run-Length Encoding (RLE) algorithm, which replaces consecutive repeated characters with a single occurrence of the character followed by a count of how many times it was repeated. For example, for the pattern aaaabccdddd, the RLE algorithm outputs a4b1c2d4.

You are given the output of the RLE algorithm for a certain pattern. Can you determine the $c$-th character of the long cipher that is formed by repeating this pattern infinitely?

Input Specification

The first line of input will consist of a string $S$, representing a pattern produced by the RLE algorithm. The length of $S$ will be at least $2$ and at most $2\cdot {10}^5$. Additionally, all numbers appearing in $S$ are between $1$ and ${10}^{12}$.

The next line of input contains a single integer $c$, representing the index of the character you wish to locate, starting from index $0$.

The following table shows how the available $15$ marks are distributed:

Marks	Bounds on $c$	Additional Constraints
6	$0\le c\le 2000$	All numbers appearing in $S$ are between $1$ and $9$ (inclusive) and the length of the repeated pattern is at most $2000$ characters.
3	$0\le c\le {10}^6$	The length of the repeated pattern is at most ${10}^6$ characters.
3	$0\le c\le {10}^{12}$	The length of the repeated pattern is at most ${10}^6$ characters.
3	$0\le c\le {10}^{12}$	No additional constraints.

Output Specification

Output the $c$-th character of the long cipher.

Sample Input 1

r2d2
8

Sample Output 1

r

Explanation for Sample Output 1

The output of the RLE algorithm r2d2 corresponds to the pattern rrdd, which creates the infinitely long cipher rrddrrddrrddrrdd..., where the $c=8$th character is r. In this example, the $c=8$th character is highlighted with a box around it.

Sample Input 2

a4b1c2d10
100

Sample Output 2

d

Explanation for Sample Output 2

The output of the RLE algorithm a4b1c2d10 corresponds to the pattern aaaabccdddddddddd. When repeated infinitely, the $c=100$th character is d.