#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
#include <fstream>
#include <utility>
#include <functional>
#include <time.h>
#include <stack>
#include <array>
#define popcount __builtin_popcount
using namespace std;
typedef long long int ll;
typedef pair<int, int> P;
const ll MOD=1e9+7;
int main()
{
    int n;
	cin>>n;
	vector<int> x0, x1;
	for(int i=0; i<n; i++){
		int x, t; cin>>t>>x; x--;
		if(t==0) x0.push_back(x);
		else x1.push_back(x);
	}
	sort(x0.begin(), x0.end());
	sort(x1.begin(), x1.end());
	int c0[3030], c1[3030];
	for(int i=0; i<n; i++){
		c0[i]=lower_bound(x0.begin(), x0.end(), i)-x0.begin();
		c1[i]=upper_bound(x1.begin(), x1.end(), i)-x1.begin();
		c0[i]=(int)x0.size()-c0[i];
		
	}
	ll dp[3030][3030]={};
	dp[0][0]=1;
	for(int i=0; i<n; i++){
		for(int j=0; j<=i; j++){
			if(c1[i]-j>0) (dp[i+1][j+1]+=dp[i][j]*(c1[i]-j))%=MOD;
			if(c0[i]-(int)x0.size()+1+i-j>0) (dp[i+1][j]+=dp[i][j]*(c0[i]-(int)x0.size()+1+i-j))%=MOD;
		}
	}
	cout<<dp[n][(int)x1.size()]<<endl;
	return 0;
}