#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <tuple>
#include <cmath>
#include <numeric>
#include <functional>
#include <cassert>
#include <atcoder/modint>

#define debug_value(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << #x << "=" << x << endl;
#define debug(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << x << endl;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

using namespace std;
typedef long long ll;

template<typename T>
vector<vector<T>> vec2d(int n, int m, T v){
    return vector<vector<T>>(n, vector<T>(m, v));
}

template<typename T>
vector<vector<vector<T>>> vec3d(int n, int m, int k, T v){
    return vector<vector<vector<T>>>(n, vector<vector<T>>(m, vector<T>(k, v)));
}

template<typename T>
void print_vector(vector<T> v, char delimiter=' '){
    if(v.empty()) {
        cout << endl;
        return;
    }
    for(int i = 0; i+1 < v.size(); i++) cout << v[i] << delimiter;
    cout << v.back() << endl;
}

using mint = atcoder::modint1000000007;

ostream& operator<<(ostream& os, const mint& m){
    os << m.val();
    return os;
}


#define N_MAX 4002
mint inv[N_MAX],fac[N_MAX],finv[N_MAX];

void init(){
    const ll MOD = mint::mod();
    fac[0]=1;fac[1]=1;
    finv[0]=1;finv[1]=1;
    inv[1]=1;
    for(int i=2;i<N_MAX;i++){
        inv[i]=mint(MOD)-inv[MOD%i]*(MOD/i);
        fac[i]=fac[i-1]*(ll) i;
        finv[i]=finv[i-1]*inv[i];
    }
}


using P = pair<int, int>;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    init();
    int n; cin >> n;
    vector<int> t(n), x(n);
    vector<P> vp;
    for(int i = 0; i < n; i++){
        cin >> t[i] >> x[i];
        if(t[i] == 1){
            x[i]--;
        }else{

        }
        vp.push_back({x[i], i});
    }
    sort(vp.begin(), vp.end());
    int cnt = 0;
    auto dp = vec2d(n+1, n+1, mint(0));
    dp[0][0] = 1;
    for(int i = 0; i < n; i++){
        auto [x, idx] = vp[i];
        for(int ng = 0; ng <= i; ng++){
            int small = x-(cnt+ng);
            if(t[idx] == 1){
                if(small >= 1) dp[i+1][ng+1] += dp[i][ng]*small;
                dp[i+1][ng] += dp[i][ng];
            }else{
                if(small >= 1) dp[i+1][ng] += dp[i][ng]*small;
            }
        }
        if(t[idx] == 0) cnt++;
    }
    mint ans = 0;
    for(int ng = 0; ng <= n; ng++){
        mint sgn = ng%2 == 0 ?  1 : -1;
        ans += sgn*dp[n][ng]*fac[n-(cnt+ng)];
    }
    cout << ans << endl;
}