二維數組寫一個二階行列式 三階不會求sum

#include<iostream>
using namespace std;
int main(){
    int arr[2][2];
    int sum;
    cout<<"|"<<"a1"<<" "<<"b1"<<"|"<<endl;
    cout<<"|"<<"a2"<<" "<<"b2"<<"|"<<endl;
    cout<<"請輸入a1:";
    cin>>arr[0][0];
    cout<<"請輸入b1:";
    cin>>arr[0][1];
    cout<<"請輸入a2:";
        cin>>arr[1][0];
        cout<<"請輸入b2:";
            cin>>arr[1][1];
                cout<<"|"<<arr[0][0]<<" "<<arr[0][1]<<"|"<<endl;
    cout<<"|"<<arr[1][0]<<" "<<arr[1][1]<<"|"<<endl;
            sum=arr[0][0]*arr[1][1]-arr[0][1]*arr[1][0];
            cout<<"sum="<<sum<<endl;


     int arrr[3][3];
     int asum;
     cout<<"| a1 b1 c1 |"<<endl;
     cout<<"| a2 b2 c2 |"<<endl;
     cout<<"| a3 b3 c3 |"<<endl;
        cout<<"請輸入a1:";
    cin>>arr[0][0];
        cout<<"請輸入b1:";
    cin>>arr[0][1];
        cout<<"請輸入c1:";
    cin>>arr[0][2];
        cout<<"請輸入a2:";
    cin>>arr[1][0]; 
    cout<<"請輸入b2:";
    cin>>arr[1][1]; 
    cout<<"請輸入c2:";
    cin>>arr[1][2]; 
    cout<<"請輸入a3:";
    cin>>arr[2][0];
        cout<<"請輸入b3:";
    cin>>arr[2][1]; 
    cout<<"請輸入c3:";
    cin>>arr[2][2];
    cout<<"|"<<arr[0][0]<<" "<<arr[0][1]<<" "<<arr[0][2]<<"|"<<endl;
        cout<<"|"<<arr[1][0]<<" "<<arr[1][1]<<" "<<arr[1][2]<<"|"<<endl;
            cout<<"|"<<arr[2][0]<<" "<<arr[2][1]<<" "<<arr[2][2]<<"|"<<endl;

} 

三階行列式的sum不知道怎麼寫。。。