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
156
157
158
159
160
161
162
163
164
165
166
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#include <stdio.h>
#include <string.h>
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#include <cassert>
using namespace std;
 
void __print(int x) {cerr << x;}
void __print(long x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(unsigned x) {cerr << x;}
void __print(unsigned long x) {cerr << x;}
void __print(unsigned long long x) {cerr << x;}
void __print(float x) {cerr << x;}
void __print(double 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...);}

#ifndef ONLINE_JUDGE
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif
#define all(x) x.begin(), x.end()
#define pb push_back
#define ll long long
#define ii pair<int,int>
#define vi vector<int>
#define vll vector<ll>
#define vii vector<ii>
#define vvi vector<vi>
#define fi first
#define se second
#define ld long double
const int MOD = 1e9 + 7;
int bin_pow(int x, int n) {
    if(n == 0) return 1;
    if(n % 2) return (1LL * x * bin_pow(x, n-1)) % MOD;
    int half = bin_pow(x, n/2);
    return (1LL * half * half) % MOD;
}
void test_case() {
    int n;
    cin >> n;
    vii v(n);
    vvi adj(n);
    for(int i = 0; i < n; i++) {
        cin >> v[i].fi >> v[i].se;    
    }
    for(int i = n - 1; i >= 0; i--) {
        int left = -1, right = -1;
        for(int j = i - 1; j >= 0; j--) {
            if(left == -1 && v[j].fi < v[i].fi && v[i].fi < v[j].se) {
                left = j;
            }
            if(right == -1 && v[j].fi < v[i].se && v[i].se < v[j].se) {
                right = j;
            }
        }
        if(left != -1) {
            adj[i].pb(left);
        }
        if(right != -1 && right != left) {
            adj[i].pb(right);
        }
    }
    vi flooded(n, 0);
    function<void(int)> flood = [&](int i) {
        flooded[i] = 1;
        for(auto v : adj[i]) {
            if(flooded[v] == 0) 
                flood(v);
        }
    };
    vi fac(n + 1, 1);
    for(int i = 1; i <= n; i++) {
        fac[i] = (1LL * fac[i-1] * i) % MOD;
    }
    vi perm(n);
    iota(all(perm), 0);
    ll ans = 0;

    do {
        for(int i = 0; i < n; i++) flooded[i] = 0;
        int cnt = 0;
        for(int i = 0; i < n; i++) {
            if(flooded[perm[i]] == 0) 
                flood(perm[i]), cnt++;
        }
        ans += cnt;

    } while(next_permutation(all(perm)));

    cout << (1LL * ans * bin_pow(fac[n], MOD - 2)) % MOD << "\n";
}
int32_t main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
 
    int tc = 1;
    // cin >> tc;
    while(tc--) {
        test_case();
    }
    return 0;
}