/*
address:https://vjudge.net/problem/Yukicoder-1001
AC 2026/10/03 11:26
*/
#include<bits/stdc++.h>
using namespace std;
const int N = 3005;
const int mod = 1e9 + 7;
int n;
int t[N], x[N];
int dp[N][N];
inline int Mod(int x) { return x >= mod ? x - mod : x; }
inline void trans(int& x, int y) { x = x + y >= mod ? x + y - mod : x + y; }
int cnt1[N], cnt2[N];
int sum1[N], sum2[N];
int C[N][N], fac[N];
int main() {
    scanf("%d", &n);
    C[0][0] = 1;
    fac[0] = 1;
    for (int i = 1;i <= n;++i) {
        fac[i] = fac[i - 1] * i % mod;
        C[i][0] = 1;
        for (int j = 1;j <= n;++j) C[i][j] = Mod(C[i - 1][j - 1] + C[i - 1][j]);
    }
    for (int i = 1;i <= n;++i) {
        scanf("%d%d", &t[i], &x[i]);
        if (t[i]) ++cnt2[x[i]];
        else ++cnt1[x[i]];
    }
    for (int i = 1;i <= n;++i)
        sum1[i] = sum1[i - 1] + cnt1[i],
        sum2[i] = sum2[i - 1] + cnt2[i];
    dp[0][0] = 1;
    for (int i = 0;i < n;++i)
        for (int j = 0;j + sum1[i] <= i;++j)
            if (dp[i][j]) {
                const int k = sum2[i + 1] - (i - j - sum1[i]);
                if (k && j >= cnt1[i + 1]) trans(dp[i + 1][j - cnt1[i + 1]], 1ll * dp[i][j] * C[j][cnt1[i + 1]] % mod * fac[cnt1[i + 1]] % mod * k % mod);
                if (j + 1 >= cnt1[i + 1]) trans(dp[i + 1][j - cnt1[i + 1] + 1], 1ll * dp[i][j] * C[j + 1][cnt1[i + 1]] % mod * fac[cnt1[i + 1]] % mod);
            }
    printf("%d", dp[n][0]);
    return 0;
}