1 #include <map>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <iostream>
 5 using namespace std;
 6 map<string, int> acc;
 7 int vis[1010], pic[1010][1010], indegree[1010];
 8 bool flag;
 9 int n, k, top; 
10 void Deal()
11 {
12         memset(vis, 0, sizeof(vis));
13         memset(pic, 0, sizeof(pic));
14         memset(indegree, 0, sizeof(indegree));
15         acc.clear(); string a, b; k = 0;
16         for(int i = 0; i < n; i++)
17         {
18             cin >> a >> b;
19             if(!acc[a]) acc[a] = ++k;
20             if(!acc[b]) acc[b] = ++k;
21             if(pic[acc[a]][acc[b]] == 0)
22             {
23                 pic[acc[a]][acc[b]] = 1;
24                 indegree[acc[b]]++;    
25             }    
26         }
27         int total = 0;    
28         for(int i = 1; i <= k; i++)
29             if(!indegree[i])
30                 total++;
31         if(total != 1)
32             flag = false;
33 }
34 void Tsort()
35 {
36     top = 0;  int i; 
37     while(top < k)
38     {
39         for(i = 1; i <= k; i++)
40             if(!vis[i] && !indegree[i])
41                 break;
42         vis[i] = 1;
43         top++;
44         for(int j = 1; j <= k; j++)
45             if(pic[i][j])
46                 indegree[j]--;
47     }
48 } 
49 
50 /*void Tsort(){                                   //WA：  不能保证没输过的只有一个人。
51     int m;
52     for(int j = 0; j < k; j++){
53         m  = -1;
54         for(int i = 1; i <= k; i++)
55             if(!indegree[i]){
56                     m = i;
57                     break;
58             }
59             if(m == -1){
60                 flag = false;
61                 break;
62             }
63             indegree[m]--;
64             for(int i = 1; i <= k; i++){
65                 if(pic[m][i])
66                     indegree[i]--;
67             }
68     }
69 } */
70 
71 int main()
72 {
73     while(~scanf("%d", &n), n)
74     {
75         flag = true;
76         Deal();
77         Tsort();
78         if(top < k)          //好像是判环。
79             flag = false;
80         if(flag)
81             printf("Yes\n");
82         else
83             printf("No\n");
84     }
85     return 0;
86 }