二位数组的两条对角线相加（与或的细节）

#include<stdio.h>
int main(){
//列0 1 2 //行
int date[4][4] = {
{ 20, 5 , 5 }//0
, { 30, 10, 20 }//1
, { 50, 20, 60 } };//2
int sum = 0,i,m;
for (i = 0; i < 3; i++){
for (m = 0; m < 3; m++) {
if (m == i||i+m==2){
sum += date[i][m];
}
}
}
printf("sum=%d\n", sum);
return 0;
}