/**
 *   @FileName	a.cpp
 *   @Author	kanpurin
 *   @Created	2020.10.17 02:44:18
**/

#include "bits/stdc++.h" 
using namespace std; 
typedef long long ll;

template< int MOD >
struct mint {
public:
    long long x;
    mint(long long x = 0) :x((x%MOD+MOD)%MOD) {}
    mint(std::string &s) {
        long long z = 0;
        for (int i = 0; i < s.size(); i++) {
            z *= 10;
            z += s[i] - '0';
            z %= MOD;
        }
        this->x = z;
    }
    mint& operator+=(const mint &a) {
        if ((x += a.x) >= MOD) x -= MOD;
        return *this;
    }
    mint& operator-=(const mint &a) {
        if ((x += MOD - a.x) >= MOD) x -= MOD;
        return *this;
    }
    mint& operator*=(const mint &a) {
        (x *= a.x) %= MOD;
        return *this;
    }
    mint& operator/=(const mint &a) {
        long long n = MOD - 2;
        mint u = 1, b = a;
        while (n > 0) {
            if (n & 1) {
                u *= b;
            }
            b *= b;
            n >>= 1;
        }
        return *this *= u;
    }
    mint operator+(const mint &a) const {
        mint res(*this);
        return res += a;
    }
    mint operator-() const {return mint() -= *this; }
    mint operator-(const mint &a) const {
        mint res(*this);
        return res -= a;
    }
    mint operator*(const mint &a) const {
        mint res(*this);
        return res *= a;
    }
    mint operator/(const mint &a) const {
        mint res(*this);
        return res /= a;
    }
    friend std::ostream& operator<<(std::ostream &os, const mint &n) {
        return os << n.x;
    }
    friend std::istream &operator>>(std::istream &is, mint &n) {
        long long x;
        is >> x;
        n = mint(x);
        return is;
    }
    bool operator==(const mint &a) const {
        return this->x == a.x;
    }
    mint pow(long long k) const {
        mint ret = 1;
        mint p = this->x;
        while (k > 0) {
            if (k & 1) {
                ret *= p;
            }
            p *= p;
            k >>= 1;
        }
        return ret;
    }
};

constexpr int MOD = 1e9 + 7;

int main() {
    int n;cin >> n;
    int cnt = 0;
    vector<pair<int,int>> a(n);
    for (int i = 0; i < n; i++) {
        int t,x;cin >> t >> x;
        if (t == 1) a[i] = {x-1,t},cnt++;
        else a[i] = {x,t};
    }
    sort(a.begin(), a.end());
    // dp[i]:iコ使うときの個数(t=0のやつは必ず使ってt=1はどっちか)
    vector<mint<MOD>> dp(n+1);
    dp[0] = 1;
    for (int i = 0; i < n; i++) {
        if (a[i].second == 0) {
            for (int j = n; j >= 1; j--) {
                dp[j] = dp[j-1] * max(0,a[i].first-j+1);
            }
            dp[0] = 0;
        }
        else {
            for (int j = n; j >= 1; j--) {
                dp[j] += dp[j-1] * max(0,a[i].first-j+1);
            }
        }
    }
    mint<MOD> ans = 0;
    vector<mint<MOD>> fact(cnt+2);
    fact[0] = fact[1] = 1;
    for (int i = 2; i <= cnt+1; i++) {
        fact[i] = fact[i-1] * i;
    }
    for (int i = 0; i <= cnt; i++) {
        if (i & 1) ans -= dp[n-cnt+i] * fact[cnt-i];
        else ans += dp[n-cnt+i] * fact[cnt-i];
    }
    cout << ans << endl;
    return 0;
}