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#include<iostream>
#include<vector>
#include<map>
#include<algorithm>
using namespace std;

const unsigned long long MOD = 1'000'000'007;

struct level {
    int a, b;
};

bool intersect(level &x, level &y) {
    if (y.a < x.a && y.b > x.a) return true;
    if (y.a < x.b && y.b > x.b) return true;
    
    return false;
}

bool intersect(level &x, int p) {
    return x.a < p && x.b > p;
}

void dfs(vector<vector<int>> &graph, vector<bool> &visited, int u) {
    visited[u] = true;
    for (int v : graph[u]) {
        if (! visited[v]) {
            dfs(graph, visited, v);
        }
    }
}

unsigned long long gcd(unsigned long long a, unsigned long long b) {
    if (a == 0) return b;
    return gcd(b % a, a);
}

unsigned long long inv(unsigned long long a) {
    return a <= 1 ? a : MOD - (MOD / a) * inv(MOD % a) % MOD;
}

unsigned long long fact(int n) {
    unsigned long long res = 1;
    for (int i = 2; i <= n; i++) {
        res *= i;
        res %= MOD;
    }
    return res;
}

unsigned long long tobits(vector<int> &nodes) {
    unsigned long long res = 0;
    for (int e : nodes) {
        res |= 1LL << e;
    }
    return res;
}

unsigned long long div(unsigned long long a, unsigned long long b) {
    unsigned long long r = gcd(a, b);
    a /= r;
    b /= r;
    return a * inv(b) % MOD;
}

unsigned long long calc(vector<vector<int>> &graph, vector<int> nodes, map<unsigned long long, unsigned long long> &m) {
    unsigned long long key = tobits(nodes);
    if (m.count(key)) {
        return m[key];
    }

    unsigned long long res = 0;
    if (nodes.size() < 2) {
        res += nodes.size();
    }
    else {
        for (int u : nodes) {
            // cout << "processing vertex " << u << " from nodes ";
            // for (int node : nodes) {
            //     cout << node << ", ";
            // }
            // cout << endl;
            vector<bool> visited(graph.size());
            dfs(graph, visited, u);
            vector<int> free;
            for (int node : nodes) {
                if (! visited[node]) {
                    free.push_back(node);
                }
            }
            
            unsigned long long s = calc(graph, free, m);
            unsigned long long t = fact(nodes.size() - 1);
            unsigned long long w = fact(free.size());
            unsigned long long accs = t + div(t, w) * s;
            // cout << "starting from " << u << " checking nodes: ";
            // for (int e : free) {
            //     cout << e << ", ";
            // }
            // cout << " result: " << accs << endl;
            res += accs;
            res %= MOD;
        }
    }
    
    m[key] = res;
    return res;
}

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

    vector<level> levels(n);
    for (int i = 0; i < n; i++) {
        cin >> levels[i].a >> levels[i].b;
    }
    
    vector<vector<int>> graph(n);
    for (int i = 1; i < n; i++) {
        for (int j = i - 1; j >= 0; j--) {
            if (intersect(levels[j], levels[i].a)) {
                graph[i].push_back(j);
                break;
            }
        }
        for (int j = i - 1; j >= 0; j--) {
            if (intersect(levels[j], levels[i].b)) {
                if (graph[i].empty() || graph[i].back() != j) {
                    graph[i].push_back(j);
                }
                break;
            }
        }
    }
    
    vector<int> perm(n);
    for (int i = 0; i < n; i++) {
        perm[i] = i;
    }
    
    unsigned long long all = 1;
    for (int i = 2; i <= n; i++) {
        all = (all * i) % MOD;
    }
    
    map<unsigned long long, unsigned long long> m;
    unsigned long long cnt = calc(graph, perm, m);
    if (cnt % all == 0) {
        cout << cnt / all << endl;
    }
    else {
        cout << div(cnt, all) << endl;
    }

    return 0;
}