import java.io.*;
/*
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 * To change this template file, choose Tools | Templates
 * and open the template in the editor.
 */

/**
 *
 * @author cteam071
 */
public class Raining {
    public static BufferedReader in;
    public static void main(String[] args) throws IOException {
        in = new BufferedReader(new InputStreamReader(System.in));
        
        String line;
        int base;
        String result = "";
        String[] pole;
        while((line = in.readLine()) != null){
            pole = line.split(" ");
            int pocet = Integer.parseInt(pole[0]);
            int[] seruje = new int[pocet];
            for (int i = 1; i < pocet +1; i++) {
                for (int j = 1; j < pocet+1; j++) {
                    if (i!=j){
                        if (pole[i].charAt(0) == pole[j].charAt(0) || pole[i].charAt(0) == pole[j].charAt(1) || 
                                pole[i].charAt(1) == pole[j].charAt(0) || pole[i].charAt(1) == pole[j].charAt(1)) {
                            seruje[i-1] += 1; 
                        }
                    }
                }
            }
            int count1 = 0;
            int count2 = 0;
            for (int i = 0; i < pocet; i++) {
                if (seruje[i] >= 2)
                    count1++;
                else if (seruje[i] == 1)
                    count2++;
            }
            if ((count2 > 2) || (count1 + count2 < pocet))
                System.out.println("NO");
            else
                System.out.println("YES");
           
        //System.out.println(result);
        }
    }
}