1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
//Sylwia Sapkowska
#include <bits/stdc++.h>
#pragma GCC optimize("O3", "unroll-loops")
using namespace std;

void __print(int x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << "'" << x << "'";}
void __print(const char *x) {cerr << '"' << x << '"';}
void __print(const string &x) {cerr << '"' << x << '"';}
void __print(bool x) {cerr << (x ? "true" : "false");}

template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ", "; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? ", " : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifdef LOCAL
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif

#define int long long
typedef pair<int, int> T;
const int oo = 1e18, oo2 = 1e9+7, K = 20;
const int mod = 1e9+7;

int mul(int a, int b){
    return (a*b)%mod;
}

void add(int &a, int b){
    a += b;
    if (a >= mod) a-=mod;
}

int power(int a, int b){
    if (!b) return 1ll;
    int k = power(a, b/2);
    k = mul(k, k);
    if (b&1) k = mul(k, a);
    return k;
}

void solve(){
    int n; cin >> n;
    vector<T>a(n+1);
    for (int i = 1; i<=n; i++) cin >> a[i].first >> a[i].second;
    set<T>s; //l,i
    a.emplace_back(0, oo2);
    s.insert({0, n+1});
    vector<vector<int>>gt(n+1), g(n+1);
    vector<int>fix;
    vector<T> nowy;
    for (int i = 1; i<=n; i++){
        auto [l, r] = a[i];
        auto it = s.upper_bound({l, oo});
        if (it != s.begin()) it--;
        fix.clear();
        nowy.assign(2, {-1, -1});
        for (; it != s.end() && it->first <= r; it++){
            fix.emplace_back(it->first);
            if (it->first < l) nowy[0] = *it;
            if (a[it->second].second > r) nowy[1] = {r, it->second};
        }
        for (auto v: fix) s.erase(s.lower_bound({v, 0}));
        for (int rep = 0; rep < 2; rep++){
            if (nowy[rep].first != -1){
                s.insert(nowy[rep]);
                g[i].emplace_back(nowy[rep].second); //moze byc rowne n
                if (nowy[rep].second <= n && !(gt[nowy[rep].second].size() > 0 && gt[nowy[rep].second].back() == i)){
                    gt[nowy[rep].second].emplace_back(i);
                } 
            }
        }
        s.insert({l, i});
        // debug(s);
    }
    vector up(2, vector<vector<int>>(K, vector<int>(n+1)));
    for (int v = 1; v<=n; v++){
        debug(v, g[v]);
        for (int rep = 0; rep < 2; rep++){
            if (g[v][rep] > n){
                for (int i = 0; i<K; i++) up[rep][i][v] = v;
            } else {
                up[rep][0][v] = g[v][rep];
                for (int i = 1; i<K; i++) up[rep][i][v] = up[rep][i-1][up[rep][i-1][v]];
            }
        }
    }
    //lewy, ma mniejszy lewy koniec
    vector<vector<int>>what(n+1);
    auto jump = [&](int f, int m, int rep){
        for (int i = K-1; i>=0; i--){
            if (up[rep][i][f] >= m){
                f = up[rep][i][f];
            }
        }
        return f;
    };
    for (int i = 1; i<=n; i++){
        if (g[i][0] > n || g[i][1] > n || g[i][0] == g[i][1]) continue;
        debug(i, g[i]);
        int left = g[i][0], right = g[i][1];
        int l = 1, r = min(left, right);
        int ret = -1;
        while (r >= l){
            int m = (l+r)/2;
            if (a[jump(left, m, 1)].second >= a[jump(right, m, 0)].first){
                ret = m;
                l = m+1;
            } else r = m-1;
        }
        if (ret != -1) {
            debug(i, ret);
            what[ret].emplace_back(i);
        }
        // while (left != right){
        //     if (left > right) {
        //         if (g[left].empty()) break;
        //         left = g[left].back();
        //     } else {
        //         if (g[right].empty()) break;
        //         right = g[right][0];
        //     }
        // }
        // if (left != right) continue;
        // debug(i, left);
        // what[left].emplace_back(i);
    }
    // return;
    vector<int>dp(n+1, 1);
    int ans = 0;
    for (int v = n; v>=1; v--){
        for (auto x: gt[v]) dp[v] += dp[x];
        for (auto x: what[v]) dp[v] -= dp[x];
        debug(v, gt[v], what[v], dp[v]);
        add(ans, power(dp[v], mod-2));
    }
    cout << ans << "\n";
}

int32_t main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    solve();

    return 0;
}