/*
Problem: Yukicoder No.1001 ???????
Time Limit: 2000 ms
Memory Limit: 512 MB
*/

#include <bits/stdc++.h>
#define nmf(i, s, e) for (int i = s; i <= e; i++)
#define ref(i, s, e) for (int i = s; i >= e; i--)
using namespace std;
typedef long long LL;
typedef unsigned long long uLL;
int cnt[3003][2];
int n;
const LL mod = 1e9 + 7;
LL f[3003][3003];
void solve()
{
    cin >> n;
    nmf(i, 1, n)
    {
        int t, x;
        cin >> t >> x;
        cnt[x][t]++;
    }
    nmf(i, 1, n) cnt[i][0] += cnt[i - 1][0];
    nmf(i, 1, n) cnt[i][1] += cnt[i - 1][1];
    f[0][0] = 1;
    nmf(a, 0, cnt[n][1])
    {
        nmf(b, 0, cnt[n][0])
        {
            int i = a + b + 1;
            if (i > n)
                continue;
            f[a + 1][b] = (f[a + 1][b] + f[a][b] * (cnt[i][1] - a)) % mod;
            f[a][b + 1] = (f[a][b + 1] + f[a][b] * (b + 1 - cnt[i - 1][0])) % mod;
        }
    }
    cout << f[cnt[n][1]][cnt[n][0]] << endl;
    return;
}
signed main()
{
    int T = 1;
    while (T--)
        solve();
    return 0;
}