#include<stdio.h>
#include<string.h>
#include<math.h>
#include<stdlib.h>
#define N 100005

int main()
{
    int a, b, k, m, i;
    bool vis[N];
    while(~scanf("%d%d", &a, &b))
    {
        k = m = 0;
        memset(vis, false, sizeof(vis));
        if(a == -1 && b == -1)
            break;
        if(a == 0 && b == 0)
        {
            printf("Yes\n");
            continue;
        }
        else
        {
              vis[a] = vis[b] = true;
              k++;
        }
        while(~scanf("%d%d", &a, &b))
        {
            if(a == 0 && b == 0)
                break;
            vis[a] = vis[b] = true;
            k++;
        }
        for(i = 0 ; i < N ; i++)
            if(vis[i])
            m++;
        if(m - k == 1)
            printf("Yes\n");
        else
            printf("No\n");
    }
    return 0;
}