#include<bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(),x.end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define INF 1000000000
#define mod 1000000007
using ll=long long;
const ll LINF=1001002003004005006ll;
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}

ll dp[3001][3001];
ll fact[3001];
signed main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    fact[0]=1;
    for(ll i=1;i<3001;i++) fact[i]=fact[i-1]*i%mod;

    int n;cin>>n;
    vector<pair<ll,ll>> v(n);
    ll cnt=0;
    rep(i,n){
        cin>>v[i].second>>v[i].first;
        if(v[i].second==1) v[i].first--,cnt++;
    }
    sort(ALL(v));

    dp[0][0]=1;
    rep(i,n){
        for(int j=0;j<=i;j++){
            dp[i+1][j]+=dp[i][j]*max(v[i].first-(i-j),0ll);
            dp[i+1][j]%=mod;
            if(v[i].second){
                dp[i+1][j+1]+=dp[i][j];
                dp[i+1][j+1]%=mod;
            }
        }
    }

    ll ans=0;
    rep(i,cnt+1){
        if(i%2==0) ans+=dp[n][cnt-i]*fact[cnt-i]%mod;
        else    ans+=mod-dp[n][cnt-i]*fact[cnt-i]%mod;
        ans%=mod;
    }
    // rep(i,n+1){
    //     rep(j,n+1){
    //         cout<<dp[i][j]<<' ';
    //     }cout<<endl;
    // }
    cout<<ans<<endl;
    return 0;
}