//#define _GLIBCXX_DEBUG
//#include "atcoder/all"
//using namespace atcoder;
#include <bits/stdc++.h>
#define int long long
#define ll long long
using ull = unsigned long long;
using namespace std;
#define Dump(x)                             \
    if (dbg) {                              \
        cerr << #x << " = " << (x) << endl; \
    }
#define overload4(_1, _2, _3, _4, name, ...) name
#define FOR1(n) for (ll i = 0; i < (n); ++i)
#define FOR2(i, n) for (ll i = 0; i < (n); ++i)
#define FOR3(i, a, b) for (ll i = (a); i < (b); ++i)
#define FOR4(i, a, b, c) for (ll i = (a); i < (b); i += (c))
#define FOR(...) overload4(__VA_ARGS__, FOR4, FOR3, FOR2, FOR1)(__VA_ARGS__)
#define FORR(i, a, b) for (int i = (a); i <= (b); ++i)
#define bit(n, k) (((n) >> (k)) & 1) /*nのk bit目*/
namespace mydef {
const int INF = 1ll << 60;
const int MOD = 1e9 + 7;
template <class T>
bool chmin(T& a, const T& b) {
    if (a > b) {
        a = b;
        return 1;
    } else
        return 0;
}
template <class T>
bool chmax(T& a, const T& b) {
    if (a < b) {
        a = b;
        return 1;
    } else
        return 0;
}
void Yes(bool flag = true) {
    if (flag)
        cout << "Yes" << endl;
    else
        cout << "No" << endl;
}
void No(bool flag = true) {
    Yes(!flag);
}
void YES(bool flag = true) {
    if (flag)
        cout << "YES" << endl;
    else
        cout << "NO" << endl;
}
void NO(bool flag = true) {
    YES(!flag);
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T& val) {
    std::fill((T*)array, (T*)(array + N), val);
}
bool dbg = true;
}  // namespace mydef
using namespace mydef;
#define pb push_back
//#define mp make_pair
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define all(v) (v).begin(), (v).end()
#define SZ(x) ((int)(x).size())
#define vi vector<int>
#define vvi vector<vector<int>>
#define vp vector<pair<int, int>>
#define vvp vector<vector<pair<int, int>>>
#define pi pair<int, int>
//#define P pair<int, int>
//#define V vector<int>
//#define S set<int>
#define asn ans

int a, b;


void solve() {
}

signed main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    cin >> a >> b;
    if (a < b)
        swap(a, b);
    Yes(a < b + 3);

    solve();
    return 0;
}

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, n) for (ll i = 0; i < (ll)(n); i++)
#define rep2(i, s, n) for (ll i = (s); i < (ll)(n); i++)
#define all(v) v.begin(), v.end()
#define sz(v) v.size()
#define INF 100000000000000 //10^14
template <typename T>
bool chmax(T &a, const T& b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <typename T>
bool chmin(T &a, const T& b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
int main()
{
    ll N;
    cin >> N;
    vector<ll> A(N), B(N);
    vector<ll> type(2*N+1, 0);
    // type.at(i)の値が-1ならi階が始点であること、0なら未確定であること、1なら終点であることを表す
    vector<vector<ll>> p(2*N+1, vector<ll>(2*N+1, 0));
    // p.at(i).at(j)の値が-1ならi階からj階への移動が不可能であること、
    // 0なら可能であること、1なら確定していることを表す
    set<ll> source; // 終点が不明な始点の集合
    set<ll> target; // 始点が不明な終点の集合
    rep(i, N){
        cin >> A.at(i) >> B.at(i);
        if(A.at(i) != -1){
            if(type.at(A.at(i)) != 0){
                cout << "No" << endl;
                return 0;
            }
            type.at(A.at(i)) = -1;
        }
        if(B.at(i) != -1){
            if(type.at(B.at(i)) != 0){
                cout << "No" << endl;
                return 0;
            }
            type.at(B.at(i)) = 1;
        }
        if(A.at(i) != -1 && B.at(i) != -1 && A.at(i) >= B.at(i)){
            cout << "No" << endl;
            return 0;
        }
        if(A.at(i) != -1 && B.at(i) != -1){
            p.at(A.at(i)).at(B.at(i)) = 1;
        }
        if(A.at(i) != -1 && B.at(i) == -1){
            source.insert(A.at(i));
        }
        if(A.at(i) == -1 && B.at(i) != -1){
            target.insert(B.at(i));
        }
    }
    rep2(i, 1, 2*N+1){
        rep2(j, 1, i+1){
            p.at(i).at(j) = -1;
        }
    }
    for(auto i : source){
        for(auto j : target){
            p.at(i).at(j) = -1;
        }
    }
    rep(i, 2*N+1){
        if(type.at(i) == -1){
            rep(j, 2*N+1){
                p.at(j).at(i) = -1;
            }
        }
        if(type.at(i) == 1){
            rep(j, 2*N+1){
                p.at(i).at(j) == -1;
            }
        }
    }
    rep(i, N){
        if(A.at(i) != -1 && B.at(i) != -1){
            rep(k, 2*N+1){
                if(k == B.at(i)){
                    continue;
                }
                p.at(A.at(i)).at(k) = -1;
            }
            rep(k, 2*N+1){
                if(k == A.at(i)){
                    continue;
                }
                p.at(k).at(B.at(i)) = -1;
            }
        }
    }
    vector<bool> dp(N+1, false);
    // dp.at(i)は最後に区切ったのが2*i階であるということが可能かどうかを表す
    dp.at(0) = true;
    bool failure = false;
    rep(i, N){
        rep(j, i+1){
            if(!dp.at(j)){
                continue;
            }
            // i+1-j階分移動することは可能か
            rep(k, i+1-j){
                if(p.at(2*j+1+k).at(j+2+i+k) == -1){
                    failure = true;
                    break;
                }
            }
            if(failure){
                failure = false;
                continue;
            }
            dp.at(i+1) = true;
            break;
        }
    }
    if(dp.at(N)){
        cout << "Yes" << endl;
    }
    else{
        cout << "No" << endl;
    }
}