#include <bits/stdc++.h>
#define int long long
#define pb push_back
using namespace std;

const int N = 3e3 + 5, mod = 1e9 + 7;
int n, f[N][N];
int a, b, C[2][N];
vector <int> S[2]; 

signed main()
{
	ios::sync_with_stdio(false);
	cin.tie(0), cout.tie(0);
	freopen(".in", "r", stdin);
	freopen(".out", "w", stdout);
	
	int i, j, t, x;
	cin >> n;
	
	for(i = 1; i <= n; i++)
	{
		cin >> t >> x;
		S[t].pb(x);
	}
	
	a = S[0].size(), b = S[1].size();
	
	for(i = 1; i <= n; i++)
	{
		for(auto p : S[0]) C[0][i] += (int)(i <= p);
		for(auto p : S[1]) C[1][i] += (int)(i >= p);
	}
	
	f[0][0] = 1;
	
	for(i = 0; i <= a; i++)
	{
		for(j = 0; j <= b; j++)
		{
			int tmp = i + j + 1;
			if(i < a) f[i + 1][j] = (f[i + 1][j] + f[i][j] * (C[0][tmp] - (a - (i + 1)))) % mod;
			if(j < b) f[i][j + 1] = (f[i][j + 1] + f[i][j] * (C[1][tmp] - j)) % mod;
		}
	}
	
	cout << f[a][b];
	return 0;
}