import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.PriorityQueue;
import java.util.Scanner;

public class Main {
	
	public static long MOD = 1000000007;
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		final int N = sc.nextInt();
		String[] fst = new String[N];
		String[] snd = new String[N];
		for(int i = 0; i < N; i++){
			fst[i] = sc.next();
		}
		for(int i = 0; i < N; i++){
			snd[i] = sc.next();
		}
		
		int answer = 0;
		for(int i = 0; i < N; i++){
			if(fst[i].equals(snd[i])){ continue; }
			
			answer = i + 1;
		}
		
		System.out.println(answer);
		for(int i = 0; i < N; i++){
			if(fst[i].equals(snd[i])){ continue; }
			System.out.println(fst[i]);
			System.out.println(snd[i]);
		}
	}
}