Uva 11906

#include <bits/stdc++.h>
#define mp make_pair
#define pb push_back
#define fi first
#define se second

#define MOD 40000
#define DOMOD(d) if ((d) >= MOD) d %= MOD;
#define DONEGMOD(d) if ((d) < 0) d = ((d % MOD) + MOD) % MOD;

#define inp(a) scanf("%d", &a)
#define inp2(a,b) scanf("%d %d", &a, &b)
#define inp3(a,b,c) scanf("%d %d %d", &a, &b, &c)
#define inp4(a,b,c,d) scanf("%d %d %d %d", &a, &b, &c, &d)

#define inpl(a) scanf("%lld", &a)
#define inpl2(a,b) scanf("%lld %lld", &a, &b)
#define inpl3(a,b,c) scanf("%lld %lld %lld", &a, &b, &c)
#define inpl4(a,b,c,d) scanf("%lld %lld %lld %lld", &a, &b, &c, &d)
#define all(v) (v).begin(), (v).end()
#define rep(i,a,b) for (int i=a;i<b;i++)
#define mset(a,val) memset(a,val,sizeof(a))
#define printv(v) for (int i=0;i<(int) v.size(); i++) cout<<v[i]<<" "
#define MAX 100005
using namespace std ;
typedef long long int ll ;
typedef pair<int,int> pii ;
typedef pair<long long , long long > pll ;



int main()
{
  #ifdef DHEER
  freopen("inp.txt","r",stdin) ;
  freopen("out.txt","w",stdout) ;
  #endif
  int n ;
  while (1)
  {
      inp(n) ;
      if (n==0) break ;
      vector<int> v[n+1] ;
      int color[n+1] ;
      int x, y ;
      while (1)
      {
          inp2(x,y) ;
          if (x==0 && y==0) break ;
          v[x].pb(y) ; v[y].pb(x) ;
      }
      for (int i=1;i<=n;i++) color[i] = -1 ;
      queue<int> q ;
      bool ans = true ;
      int f ;
      for (int i=i;i<=n;i++)
      {
          if (color[i] == -1)
          {
              q.push(i) ;
              color[i] = 0 ;
              while (!q.empty())
              {
                  f = q.front() ;
                  q.pop() ;
                  for (auto it : v[f])
                  {
                      if (color[it] == -1)
                      {
                          color[it] = 1 - color[f] ;
                          q.push(it) ;
                      }else if (color[it] == color[f]) {
                          ans = false ; break ;
                      }
                  }
                  if (!ans) break ;
              }
              if (!ans) break ;
          }

      }
      if (ans) cout<<"YES"<<endl ;
      else cout<<"NO"<<endl ;

  }
    return 0 ;
}