1 import java.util.*;
  2 import java.io.*;
  3
  4 class FastReader{
  5     BufferedReader reader;
  6     StringTokenizer tokenizer;
  7
  8     public FastReader(InputStream stream){
  9         reader = new BufferedReader(new InputStreamReader(stream), 1 << 22);
 10         tokenizer = null;
 11     }
 12
 13     public String next(){
 14         while (tokenizer == null || !tokenizer.hasMoreTokens()){
 15             try {
 16                 tokenizer = new StringTokenizer(reader.readLine());
 17             } catch (Exception e){
 18                 //System.out.println(-1);
 19                 throw new RuntimeException(e);
 20             }
 21         }
 22
 23         return tokenizer.nextToken();
 24     }
 25
 26     public int next_int(){
 27         return Integer.parseInt(next());
 28     }
 29 }
 30
 31 class Node{
 32     int address;
 33     int next_address;
 34     Node next;
 35
 36     public Node(){
 37         next = null;
 38     }
 39 }
 40
 41 public class Main {
 42     public static void main(String[] args){
 43         FastReader reader = new FastReader(System.in);
 44         int first_start, second_start;
 45         int N;
 46
 47         first_start = reader.next_int();
 48         second_start = reader.next_int();
 49         N = reader.next_int();
 50
 51         Node[] nodes = new Node[N];
 52
 53         for (int i = 0; i < N; i++){
 54             int cur_address = reader.next_int();
 55             reader.next();
 56             int next_address = reader.next_int();
 57
 58             Node node = new Node();
 59             node.address = cur_address;
 60             node.next_address = next_address;
 61             nodes[i] = node;
 62         }
 63
 64         Node first_word = null, second_word = null;
 65         for (int i = 0; i < N; i++){
 66             Node cur_node = nodes[i];
 67             if (cur_node.address == first_start)
 68                 first_word = cur_node;
 69             else if (cur_node.address == second_start)
 70                 second_word = cur_node;
 71             for (int j = 0; j < N; j++){
 72                 Node candidate_node = nodes[j];
 73
 74                 if (cur_node.next_address == candidate_node.address){
 75                     cur_node.next = candidate_node;
 76                     break;
 77                 }
 78             }
 79         }
 80
 81         int len1 = 0, len2 = 0;
 82         Node first_ptr = first_word, second_ptr = second_word;
 83
 84         if (first_ptr == null || second_ptr == null){
 85             System.out.println(-1);
 86             return;
 87         }
 88
 89         while (first_ptr.next != null){
 90             first_ptr = first_ptr.next;
 91             len1++;
 92         }
 93         while (second_ptr.next != null){
 94             second_ptr = second_ptr.next;
 95             len2++;
 96         }
 97
 98         if (first_ptr != second_ptr){
 99             System.out.println(-1);
100             return;
101         }
102
103         int diff_step = Math.max(len1, len2) - Math.min(len1, len2);
104         if (len1 < len2){
105             first_ptr = second_word;
106             second_ptr = first_word;
107         } else {
108             first_ptr = first_word;
109             second_ptr = second_word;
110         }
111
112         for (int i = 0; i < diff_step; i++)
113             first_ptr = first_ptr.next;
114         while (first_ptr != second_ptr){
115             first_ptr = first_ptr.next;
116             second_ptr = second_ptr.next;
117         }
118
119         System.out.println(first_ptr.address);
120     }
121 }