#include <bits/stdc++.h>
using namespace std;

const int64_t MOD = 1e9+7;
void add(int64_t& a, int64_t b){
    a = (a+b) % MOD;
}
void mul(int64_t& a, int64_t b){
    a = a*b % MOD;
}

vector<int64_t> fact, seq_inv, fact_inv;

void create_fact_mod(int num){
    fact[0] = fact[1] = 1;
    for(int i=2; i<=num; i++) fact[i] = fact[i-1] * i % MOD;
}

void create_seq_inv_mod(int num){
    seq_inv[0] = seq_inv[1] = 1;
    for(int i=2; i<=num; i++) seq_inv[i] = (MOD - MOD/i) * seq_inv[MOD%i] % MOD;
}

void create_fact_inv_mod(int num){
    fact_inv[0] = fact_inv[1] = 1;
    for(int i=2; i<=num; i++) fact_inv[i] = fact_inv[i-1] * seq_inv[i] % MOD;
}

void create_mod_tables(int num){
    fact.resize(num+1);
    seq_inv.resize(num+1);
    fact_inv.resize(num+1);
    create_fact_mod(num);
    create_seq_inv_mod(num);
    create_fact_inv_mod(num);
}

int64_t comb_mod(int n, int k){
    return fact[n] * fact_inv[n-k] % MOD * fact_inv[k] % MOD;
}

int64_t perm_mod(int n, int k){
    return fact[n] * fact_inv[n-k] % MOD;
}

int64_t power_mod(int64_t num, int64_t power){
    int64_t prod = 1;
    num %= MOD;
    while(power > 0){
        if(power&1) prod = prod * num % MOD;
        num = num * num % MOD;
        power >>= 1;
    }
    return prod;
}

int64_t extgcd(int64_t a, int64_t b, int64_t& x, int64_t& y){
    int64_t d = a;
    if(b != 0){
        d = extgcd(b, a%b, y, x);
        y -= (a/b) * x;
    }else{
        x = 1; y = 0;
    }
    return d;
}

int64_t inv_mod(int64_t a){
    int64_t x, y;
    extgcd(a, MOD, x, y);
    return (MOD + x%MOD) % MOD;
}

int main(){
    int N;
    cin >> N;
    vector<int> lb, ub;
    for(int i=0; i<N; i++){
        int t, x;
        cin >> t >> x;
        x--;
        if(t == 1){
            lb.push_back(x);
        }else{
            ub.push_back(x);
        }
    }
    int X = lb.size(), Y = ub.size();
    sort(lb.rbegin(), lb.rend());
    sort(ub.rbegin(), ub.rend());

    int cx = 0, cy = Y;

    static int64_t dp[3010][3010];
    dp[0][0] = 1;
    for(int i=0; i<N; i++){
        while(lb.size() && lb.back() <= i) cx++, lb.pop_back();
        while(ub.size() && ub.back() < i) cy--, ub.pop_back();
        for(int x=0; x<=min(i, X); x++){
            int y = i-x;
            if(y > Y) continue;
            if(cx > x) add(dp[x+1][y], dp[x][y] * (cx-x));
            if(cy > Y-y-1) add(dp[x][y+1], dp[x][y] * (cy-(Y-y-1)));
        }
    }
    cout << dp[X][Y] << endl;
    return 0;
}