Demo Exam(Deciphering)

package Demo0604Exam;

import java.util.Scanner;

public class proba {

        public static void main(String[] args) {
            Scanner scanner = new Scanner(System.in);

            String input = scanner.nextLine();

            String[] substringArr = scanner.nextLine().split("\\s+");

            for (int i = 0; i < input.length(); i++) {
                int letter = input.charAt(i);
                if(letter != 35) {
                    if (letter < 100 || letter > 125) {
                        System.out.println("This is not the book you are looking for.");
                        return;
                    }
                }
            }

            String word = "";
            for (int i = 0; i < input.length(); i++) {
                int letter = input.charAt(i);
                int letterF = letter - 3;
                word += (char)letterF;
            }

                word = word.replace(substringArr[0], substringArr[1]);
                System.out.println(word);

        }
    }