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#include <bits/stdc++.h>
#pragma GCC optimize("O3", "unroll-loops")
#define pb emplace_back
#define ins insert
#define mp make_pair
#define ssize(x) (int)x.size()
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define pii pair <int, int>
#define pll pair <ll, ll>
#define pld pair <ld, ld>
#define st first
#define nd second

using namespace std;
using ll = int_fast64_t;
// using lll = __int128_t;
using ld = long double;
const int oo = 1e9 + 7;
const ll ool = 1e18;
const int mod = 1e9 + 7;

void solve(){
    int n; cin >> n;
    vector <pii> tab(n);
    for(auto &[x, y]: tab) cin >> x >> y;
    vector <ll> cnt(n);
    vector <vector<int>> g(n);
    for(int i = 1; i < n; i ++){
        int l = tab[i].st, r = tab[i].nd;
        for(int j = i - 1; j >= 0; j --){
            if(l != -1){
                if(tab[j].st <= l && tab[j].nd >= l){
                    g[j].pb(i);
                    l = -1;
                }
            }
            if(r != -1){
                if(tab[j].st <= r && tab[j].nd >= r){
                    g[j].pb(i);
                    r = -1;
                }
            }
        }
    }
    vector <bool> vis(n);
    function <void(int)> dfs = [&](int v){
        vis[v] = 1;
        for(auto u: g[v]){
            if(!vis[u]) dfs(u);
        }
    };
    for(int i = 0; i < n; i ++){
        dfs(i);
        for(int j = 0; j < n; j ++){
            if(vis[j]) ++ cnt[i];
        }
        -- cnt[i];
        fill(all(vis), 0);
    }
    vector <ll> fact(n + 3, 1);
    for(ll i = 1; i <= n; i ++) fact[i] = (fact[i - 1] * i) % mod;
    auto fp = [&](ll a, ll b){
        ll r = 1;
        while(b){
            if(b & 1) r = (r * a) % mod;
            a = (a * a) % mod;
            b >>= 1;
        }
        return r;
    };
    ll ans = 0;
    for(int i = 0; i < n; i ++){
        ll rn = 0;
        for(ll j = 0; j < n; j ++){
            if(cnt[i] > n - j - 1){
                rn += fact[n - 1];
                rn %= mod;
                continue;
            }
            ll x = (fact[n - j - 1] * fp(fact[n - j - 1 - cnt[i]], mod - 2)) % mod;
            x = (x * fact[n - 1 - cnt[i]]) % mod;
            rn += (fact[n - 1] - x + mod) % mod;
            rn %= mod;
        }
        ans += (fact[n] - rn + mod) % mod;
        ans %= mod;
    }
    cout << (ans * fp(fact[n], mod - 2)) % mod << '\n';

}

signed main(){
    ios_base::sync_with_stdio(0); cin.tie(0);
    int t; t = 1;
    // cin >> t;
    while(t --) solve();
    return 0;
}