#define  _CRT_SECURE_NO_WARNINGS
#pragma comment (linker, "/STACK:526000000")

#include "bits/stdc++.h"

using namespace std;
typedef string::const_iterator State;
#define eps 1e-11L
#define MAX_MOD 1000000007LL
#define GYAKU 500000004LL

#define MOD 998244353LL
#define seg_size 262144
#define pb push_back
#define mp make_pair
typedef long long ll;
#define REP(a,b) for(long long (a) = 0;(a) < (b);++(a))
#define ALL(x) (x).begin(),(x).end()

unsigned long xor128() {
	static unsigned long x = 123456789, y = 362436069, z = 521288629, w = time(NULL);
	unsigned long t = (x ^ (x << 11));
	x = y; y = z; z = w;
	return (w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)));
}

void init() {
	iostream::sync_with_stdio(false);
	cout << fixed << setprecision(20);
}

#define int ll
int dp[3001][3001] = {};
int limit_lower[3002];
int limit_higher[3002];
void solve() {
	int n;
	cin >> n;
	REP(i, n) {
		int a, b;
		cin >> a >> b;
		if (a == 0) {
			limit_lower[b]++;
		}
		else {
			limit_higher[b]++;
		}
	}
	for (int i = 0; i < 3001; ++i) {
		limit_higher[i + 1] += limit_higher[i];
		limit_lower[i + 1] += limit_lower[i];
	}
	dp[0][0] = 1;
	for (int i = 1; i <= n; ++i) {
		for (int q = 0; q <= limit_higher[i-1]; ++q) {
			if (dp[i-1][q] == 0) continue;
			//no take
			dp[i][q] += dp[i - 1][q];
			dp[i][q] %= MAX_MOD;
			
			//take right
			dp[i][q + 1] += (dp[i - 1][q] * (limit_higher[i] - q)) % MAX_MOD;
			dp[i][q + 1] %= MAX_MOD;
		}
		for (int q = 0; q <= limit_higher[i]; ++q) {
			if (dp[i][q] == 0) continue;
			int now_have = i - q;
			now_have -= limit_lower[i - 1];
			for (int j = 0; j < limit_lower[i] - limit_lower[i-1]; ++j) {
				dp[i][q] *= now_have;
				dp[i][q] %= MAX_MOD;
				now_have--;
			}
			dp[i][q] %= MAX_MOD;
		}
	}
	cout << dp[n][limit_higher[n]] << endl;
}

#undef int
int main() {
	init();
	solve();
}