1 #include<cstdio>
  2 #include<iostream>
  3 #include<cstring>
  4 #include<string>
  5 #include<algorithm>
  6 #include<map>
  7 using namespace std;
  8 
  9 map<string,int>id;
 10 int G[1005][1005];
 11 bool indegree[1005];
 12 int num;
 13 
 14 bool toposort()
 15 {
 16     int cnt,i;
 17     for(cnt=i=0;i<num;i++)
 18     {
 19         if(indegree[i])
 20         cnt++;
 21     }
 22     if(cnt!=num-1)return false;
 23     return true;
 24 }
 25 
 26 int main()
 27 {
 28     //freopen("in.txt","r",stdin);
 29     int n;
 30     string s1,s2;
 31     while(scanf("%d",&n),n)
 32     {
 33         num=0;
 34         id.clear();
 35         memset(indegree,0,sizeof(indegree));
 36         memset(G,0,sizeof(G));
 37         for(int i=0;i<n;i++)
 38         {
 39             cin>>s1>>s2;
 40             if(!id.count(s1))
 41                 id[s1]=num++;
 42             if(!id.count(s2))
 43                 id[s2]=num++;
 44                 if(!G[id[s1]][id[s2]])
 45                     indegree[id[s2]]=true;
 46             G[id[s1]][id[s2]]=1;
 47         }
 48         bool ans=toposort();
 49         if(ans)
 50             printf("Yes\n");
 51         else
 52             printf("No\n");
 53     }
 54     return 0;
 55 }
 56 
 57 
 58 /*判断图有唯一拓扑序列
 59 #include<cstdio>
 60 #include<iostream>
 61 #include<cstring>
 62 #include<string>
 63 #include<algorithm>
 64 #include<map>
 65 using namespace std;
 66 
 67 map<string,int>id;
 68 int G[1005][1005];
 69 bool indegree[1005];
 70 int num;
 71 int c[1005];
 72 
 73 bool dfs(int u)
 74 {
 75     c[u]=-1;
 76     for(int v=0;v<num;v++)
 77     {
 78         if(G[u][v])
 79         {
 80             if(c[v]<0)return false;
 81             else if(!c[v]&&!dfs(v))return false;
 82         }
 83     }
 84     c[u]=1;
 85     return true;
 86 }
 87 
 88 bool toposort()
 89 {
 90     int flag;
 91     int cnt,i;
 92     memset(c,0,sizeof(c));
 93     for(cnt=i=0;i<num;i++)
 94     {
 95         if(indegree[i]==false)
 96             flag=i;
 97         else
 98             cnt++;
 99     }
100     if(cnt!=num-1)return false;
101     if(!dfs(flag))return false;
102     return true;
103 }
104 
105 int main()
106 {
107     //freopen("in.txt","r",stdin);
108     int n;
109     string s1,s2;
110     while(scanf("%d",&n),n)
111     {
112         num=0;
113         id.clear();
114         memset(indegree,0,sizeof(indegree));
115         memset(G,0,sizeof(G));
116         for(int i=0;i<n;i++)
117         {
118             cin>>s1>>s2;
119             if(!id.count(s1))
120                 id[s1]=num++;
121             if(!id.count(s2))
122                 id[s2]=num++;
123                 if(!G[id[s1]][id[s2]])
124                     indegree[id[s2]]=true;
125             G[id[s1]][id[s2]]=1;
126         }
127         bool ans=toposort();
128         if(ans)
129             printf("Yes\n");
130         else
131             printf("No\n");
132     }
133     return 0;
134 }
135 */