#include <iostream>
#include <vector>
#include <algorithm>
#include <utility>

using namespace std;

using ll =  long long;
using Pii = pair<int, int>;

constexpr ll MOD = 1000000007;

int main() {
    int n;
    cin >> n;
    vector<Pii> p(n);
    int min_cnt = 0;
    for(int i=0;i<n;++i) {
        cin >> p[i].second >> p[i].first;
        if(p[i].second == 1) {
            --p[i].first;
        } else {
            ++min_cnt;
        }
    }
    sort(p.begin(), p.end());

    vector<vector<ll>> dp(n+1, vector<ll>(n+1, 0));
    dp[0][0] = 1;
    for(int i=0;i<n;++i) {
        for(int j=0;j<=i;++j) {
            if(p[i].second == 1) {
                dp[i+1][j] += dp[i][j];
                dp[i+1][j] %= MOD;
            }
            dp[i+1][j+1] += dp[i][j] * max(0, p[i].first-j) % MOD;
            dp[i+1][j+1] %= MOD;
        }
    }

    ll fact[n+1];
    fact[0] = 1;
    for(int i=1;i<=n;++i) fact[i] = (fact[i-1] * i) % MOD;

    ll ans = 0LL;
    for(int i=min_cnt;i<=n;++i) {
        if((i-min_cnt) % 2) {
            ans += MOD - dp[n][i] * fact[n-i] % MOD;
        } else {
            ans += dp[n][i] * fact[n-i] % MOD;
        }
        ans %= MOD;
    }
    
    cout << ans << endl;
}