#include <iostream>

 #include <cstring>

 #include <cstdlib>

 #include <cstdio>

 #include <numeric>

 #include <algorithm>

 #include <cctype>

 #include <string>

 using namespace std;

 const int MAXN = ;

 bool flag[MAXN];

 int connectedList[MAXN];

 bool isOk = true;

 int nStart, nEnd;

 void Init() {

     for (int i = ; i < MAXN; ++i) {

         connectedList[i] = i;

         flag[i] = false;

     }

 }

 int getFather(int vetex) {

     int pos = vetex;

     while(pos != connectedList[pos]) {

         pos = connectedList[pos];

     }

     return pos;

 }

 int countListNum() {

     int count = ;

     for (int i = ; i < MAXN; ++i) {

         if (flag[i] == true) {

             if (getFather(i) == i) {

                 ++count;

             }

         }

     }

     return count;

 }

 void process() {

     if( getFather(nStart) == getFather(nEnd)) {

         isOk = false;

         return;

     }

     if( getFather(nStart) == nStart && getFather(nEnd) == nEnd) {

         connectedList[nEnd] = nStart;

     } else if(getFather(nStart) == nStart) {

         connectedList[nStart] = getFather(nEnd);

     } else {

         connectedList[nEnd] = getFather(nStart);

     }

     flag[nStart] = true;

     flag[nEnd] = true;

 }

 int main() {

     while(cin >> nStart >> nEnd) {

         if(nStart == - && nEnd == -) break;

         if(nStart || nEnd) {

             isOk = true;

             memset(flag, false , sizeof(flag));

             Init();

             process();

             while(cin >> nStart >> nEnd, nStart || nEnd) {

                 if(isOk == true) {

                     process();

                 }

             }

             if(isOk == true) {

                 int count = ;

                 count = countListNum();

                 if(count > ) {

                     isOk = false;

                 }

             }

         } else {

             isOk = true;

         }

         if(isOk == false) {

             cout << "No" << endl;

         } else {

             cout << "Yes" << endl;

         }

     }

     return ;

 }