求矩形正、副对角线之和

#include<stdio.h>
int main()
{
int arr[3][3] = {1,2,3,4,5,6,7,8,9};
int i = 0,j = 0,sum = 0, sum1 =0;
for(i = 0;i < 3;i ++)
{
for(j = 0;j < 3; j ++)
{
if(i == j )
    {
       sum = sum +arr[i][j];
    }
    if(i +j == 2 && i != j)
    {
      sum1 = sum1 + arr[i][j];
    }
  }
}
int num = 0;
num = sum + sum1;
printf("%d\n",num);
return 0;
}