#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;

int main() {
    int N;
    cin >> N;
    vector<int> V[2]; // V[0]: t=0, V[1]: t=1
    for (int i = 0; i < N; i++) {
        int t, x;
        cin >> t >> x;
        V[t].push_back(x - 1); // ?? 0-indexed
    }
    // ????????
    sort(V[0].begin(), V[0].end());
    sort(V[1].begin(), V[1].end());
    
    int n0 = V[0].size(), n1 = V[1].size();
    vector<vector<long long>> dp(n0 + 1, vector<long long>(n1 + 1, 0));
    dp[0][0] = 1;
    
    for (int i = 0; i < N; i++) { // i = ???????
        // ??? cnt0, cnt1
        int cnt0 = 0, cnt1 = 0;
        for (int v : V[0]) cnt0 += (v >= i);
        for (int v : V[1]) cnt1 += (i >= v);
        
        for (int x = 0; x <= n0; x++) {
            int y = i - x;
            if (y < 0 || y > n1) continue;
            if (dp[x][y] == 0) continue;
            
            // ?? t=1 ???
            if (y < n1) {
                long long ways = cnt1 - y;
                if (ways > 0) {
                    dp[x][y + 1] = (dp[x][y + 1] + dp[x][y] * ways) % MOD;
                }
            }
            // ?? t=0 ???
            if (x < n0) {
                long long ways = cnt0 - (n0 - (x + 1));
                if (ways > 0) {
                    dp[x + 1][y] = (dp[x + 1][y] + dp[x][y] * ways) % MOD;
                }
            }
        }
    }
    
    long long ans = 0;
    for (int x = 0; x <= n0; x++) {
        int y = N - x;
        if (y >= 0 && y <= n1) {
            ans = (ans + dp[x][y]) % MOD;
        }
    }
    cout << ans << endl;
    return 0;
}