#include <bits/stdc++.h>
 
//#include <unistd.h>
//#include <iostream>
 
using namespace std;
 
#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl;
#define DEBUG_MAT(v) cerr<<#v<<endl;for(int i=0;i<v.size();i++){for(int j=0;j<v[i].size();j++) {cerr<<v[i][j]<<" ";}cerr<<endl;}
typedef long long ll;
//#define int ll
 
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
template<class S, class T> pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first + t.first, s.second + t.second); }
template<class S, class T> pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first - t.first, s.second - t.second); }
template<class S, class T> ostream& operator<<(ostream& os, pair<S, T> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
#define X first
#define Y second
#define rep(i,n) for(int i=0;i<(n);i++)
#define rep1(i,n) for(int i=1;i<=(n);i++)
#define rrep(i,n) for(int i=(n)-1;i>=0;i--)
#define rrep1(i,n) for(int i=(n);i>0;i--)
#define REP(i,a,b) for(int i=a;i<b;i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(),c.end()
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a = b; return 1; } return 0; }
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = (ll)1e18 + 1;
const long double pi = 3.1415926535897932384626433832795028841971L;
#define Sp(p) cout<<setprecision(25)<< fixed<<p<<endl;
//int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
//int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };
vi dx = {1, 0, -1, 0}, dy = {0, 1, 0, -1};
vi dx2 = { 1,1,0,-1,-1,-1,0,1 }, dy2 = { 0,1,1,1,0,-1,-1,-1 };
#define fio() cin.tie(0); ios::sync_with_stdio(false);
const ll MOD = 1000000007;
//const ll MOD = 998244353;
// #define mp make_pair
//#define endl '\n'


signed main() {
	int n;
	cin >> n;
	vector<pii> xt(n);
	rep (i, n) {
		cin >> xt[i].second >> xt[i].first;
		xt[i].first--;
		if (xt[i].second == 1) xt[i].first--;
	}
	sort(all(xt));
	//DEBUG_VEC(xt);

	vll dp(n + 1, vl(n + 1));
	dp[0][0] = 1;
	int one = 0;
	rep (i, n) {
		rep (j, n + 1) {
			if (dp[i][j] == 0) continue;
			ll x = xt[i].first;
			if (xt[i].second == 0) {
				dp[i + 1][j] += dp[i][j] * (x + 1 - (i - (one - j))) % MOD;
				dp[i + 1][j] %= MOD;
			}
			else {
				if (j < n and x + 1 - (i - (one - j)) >= 0) {
					dp[i + 1][j + 1] += dp[i][j] * (x + 1 - (i - (one - j))) % MOD;
					dp[i + 1][j + 1] %= MOD;
				}
				dp[i + 1][j] += dp[i][j];
				dp[i + 1][j] %= MOD;
			}
		}
		if (xt[i].second == 1) {
			one++;
		}
	}
	vl fact(n + 1);
	fact[0] = 1;
	rep (i, n) {
		fact[i + 1] = fact[i] * (i + 1) % MOD;
	}

	rep (j, n + 1) {
		if (dp[n][j] != 0) {
			dp[n][j] = dp[n][j] * fact[one - j];
		}
	}

	ll ans = 0;
	rep (j, n + 1) {
		if (j % 2 == 0) ans += dp[n][j];
		else ans -= dp[n][j];
	}
	ans = (ans % MOD + MOD) % MOD;
	cout << ans << endl;
}