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
#include <algorithm>
#include <cmath>
#include <iostream>
#include <vector>

const int MOD = 1000000007;

    long long inv(long long q) {
        long long p = 1000000007;
        long long res = 1;
        long long exp = p - 2;
        while (exp > 0) {
            if (exp % 2 == 1) {
                res = (res * q) % p;
            }
            q = (q * q) % p;
            exp /= 2;
        }
        return res;
    }

int main() {
    int n;
    std::cin >> n;

    std::vector<int> l(n), r(n);
    for (int i = 0; i < n; i++) {
        std::cin >> l[i] >> r[i];
    }

    std::vector<std::vector<int>> zaleznosc(n);
    for (int i = 0; i < n-1; i++) {
        for (int j = i+1; j < n; j++) {
            if (l[i] >= l[j] && l[i] <= r[j]) {
                zaleznosc[i].push_back(j);
                break;
            }
        }
        for (int j = i+1; j < n; j++) {
            if (r[i] >= l[j] && r[i] <= r[j]) {
                zaleznosc[i].push_back(j);
                break;
            }
        }
    }

    std::vector<bool> visited(n, false);
    long long sum = 0;
    std::vector<int> nums(n);
    for (int i = 0; i < n; i++) {
        nums[i] = i;
    }

    int coutner = 0;
    int coun =0;
    do {
        visited.assign(n, false);
        coun++;
        for (int i = 0; i < nums.size(); i++) {
            if (!visited[nums[i]]) {
                coutner++;
            }
            for (int j = 0; j < zaleznosc[nums[i]].size(); j++) {
                visited[zaleznosc[nums[i]][j]] = true;
            }
        }
        sum += coutner;
        coutner = 0;

    } while (std::next_permutation(nums.begin(), nums.end()));


    long long p = sum % MOD;
    long long q = coun % MOD;
    long long result = (p * inv(q)) % MOD;

    std::cout << result << std::endl;


    return 0;
}