#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> l_l;
typedef pair<int, int> i_i;
template<class T>
inline bool chmax(T &a, T b) {
    if(a < b) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
inline bool chmin(T &a, T b) {
    if(a > b) {
        a = b;
        return true;
    }
    return false;
}

const long double EPS = 1e-10;
const long long INF = 1e18;
const long double PI = acos(-1.0L);
const ll mod = 1000000007;
ll N;
vector<ll> t, X;
ll dp[3002][3002];

int main() {
    //cout.precision(10);
    cin.tie(0);
    ios::sync_with_stdio(false);
    cin >> N;
    t.resize(N);
    X.resize(N);
    for(int i = 0; i < N; i++) {
        cin >> t[i] >> X[i];
        X[i]--;
    }
    dp[0][0] = 1;
    ll Up = 0;
    ll Down = 0;
    for(ll val = 0; val < N; val++) {
        for(int i = 0; i < N; i++) {
            if(t[i] == 1 and X[i] == val) Up++;
        }
        for(int before = 0; before <= N; before++) {
            ll delta = Up - before;
            chmax(delta, 0LL);
            dp[val+1][before+1] += dp[val][before] * delta;
            dp[val+1][before+1] %= mod;
            dp[val+1][before] += dp[val][before];
            dp[val+1][before] %= mod;
        }
        for(int i = 0; i < N; i++) {
            if(t[i] == 0 and X[i] == val) {
                for(ll before = 0; before <= N; before++) {
                    ll delta = val + 1 - before - Down;
                    if(delta <= 0) dp[val+1][before] = 0;
                    else dp[val+1][before] *= delta;
                    dp[val+1][before] %= mod;
                }
                Down++;
            }
        }
    }
    ll ans = dp[N][Up];
    /*
    for(int i = 0; i <= N; i++) {
        for(int j = 0; j <= N; j++) {
            cerr << dp[i][j] << " ";
        }
        cerr << endl;
    }
    */
    cout << ans << endl;
    return 0;
}