// 参考文献 : http://kmjp.hatenablog.jp/entry/2020/02/29/0930

#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <tuple>
#include <deque>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <complex>
using namespace std;

typedef long long ll;
typedef pair<int, int> P;
typedef pair<int, double> Pid;
typedef pair<double, int> Pdi;
typedef pair<ll, int> Pl;
typedef pair<int, pair<int, int>> PP;
const double PI = 3.1415926535897932;   // acos(-1)
const double EPS = 1e-15;
const int INF = 1001001001;
const int mod = 1e+9 + 7;

#define chmax(x, y) x = max(x, y)
#define chmin(x, y) x = min(x, y)
#define chadd(x, y) x = (x + y) % mod

int n;
vector<int> input[2];
ll dp[3050][3050];

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    cin >> n;
    for(int i = 0; i < n; ++i){
        int x, y;
        cin >> x >> y;
        input[x].push_back(--y);
    }

    dp[0][0] = 1;
    int sz0 = input[0].size(), sz1 = input[1].size();
    for(int i = 0; i < n; ++i){
        int cnt0 = 0, cnt1 = 0;
        for(int j : input[0])   cnt0 += j >= i;
        for(int j : input[1])   cnt1 += i >= j;
        for(int j = 0; j <= sz0; ++j){
            int k = i - j;
            if(k < 0 || k > sz1)    continue;
            if(j < sz0)     chadd(dp[j+1][k], dp[j][k] * (cnt0 - (sz0 - j - 1)));
            if(k < sz1)     chadd(dp[j][k+1], dp[j][k] * (cnt1 - k));
        }
    }
    ll res = 0;
    for(int i = 0; i <= n; ++i)     chadd(res, dp[i][n-i]);
    cout << res << endl;
}