import java.util.*;
import java.util.regex.*;

class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		String fileName = "20210101-000000_first_second_1234.csv";
		Pattern pattern = Pattern.compile("\\d{4}(?=\\.[^.]+$)");
		Matcher m = pattern.matcher(fileName);
		if (m.find()) {
			System.out.println(m.group());
		}
	}
}