1 #include<stdio.h>
2 #include<string.h>
3 int main()
4 {
5 int compair(char a[],char b[]);
6 int n,i,j,t,frist;
7 struct
8 {
9 char a[100];
10 char b[100];
11 } grade[1000];
12 while(~scanf("%d",&n)&&n)
13 {
14 memset(grade,0,sizeof grade);
15 for(i=0; i<n; ++i)
16 scanf("%s%s",grade[i].a,grade[i].b);
17 for(i=t=0; i<n; ++i)
18 {
19 for(j=0; j<n; ++j)
20 if(compair(grade[i].a,grade[j].b))
21 break;
22 if(j==n)
23 if(t)
24 {
25 if(!compair(grade[frist].a,grade[i].a))
26 ++t;
27 }
28 else
29 {
30 frist=i;
31 ++t;
32 }
33 //printf("t=%d\n",t);
34 if(t>1)
35 {
36 printf("No\n");
37 break;
38 }
39 }
40 if(t==1)
41 printf("Yes\n");
42 else if(t<1)
43 printf("No\n");
44 }
45 }
46
47 int compair(char a[],char b[])
48 {
49 int i;
50 for(i=0; a[i]!='\0'&&b[i]!='\0'; ++i)
51 if(a[i]!=b[i])
52 {
53 return 0;
54 break;
55 }
56 if(a[i]!='\0'||b[i]!='\0')
57 return 0;
58 else
59 return 1;
60 }
