#include <cstdio>
#include <cstring>
#include <algorithm>
#include <map>
#define MP(a, b) make_pair(a, b)
using namespace std;
const int MAXN = 100000 + 5;
const int INF = 0x3f3f3f3f;
 
int pa[MAXN], vis[MAXN];
 
int Find(int x)
{
    return pa[x] == x ? x : pa[x] = Find(pa[x]);
}
 
int main()
{
    //freopen("input.txt", "r", stdin);
    int a, b, i, j;
    while (scanf("%d%d", &a, &b), a + b != -2)
    {
        if (a + b == 0)
        {
            printf("Yes\n");
            continue;
        }
        bool flag = false;
        for (i = 1; i <= MAXN; i++)
        {
            pa[i] = i;
            vis[i] = 0;
        }
        vis[a] = vis[b] = 1;
        int x = Find(a), y = Find(b);
        pa[x] = y;
        while (scanf("%d%d", &a, &b), a + b)
        {
            if (flag)
                continue;
            x = Find(a), y = Find(b);
            if (x == y)
                flag = true;
            else
            {
                vis[a] = vis[b] = 1;
                pa[x] = y;
            }
        }
        if (flag)
        {
            printf("No\n");
            continue;
        }
        int cnt = 0;
        bool flgin = false;
        for (i = 1; i < MAXN; i++)
            if (vis[i] && pa[i] == i)
            {
                cnt++;
                if (cnt > 1)
                {
                    flgin = true;
                    break;
                }
            }
        printf("%s\n", flgin ? "No" : "Yes");
    }
    return 0;
}