Summing across diagonals and abs value of subtracting diagonal sums

sumDiagonal.cpp

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;


int main(){
    int n;
    cin >> n;
    vector< vector<int> > a(n,vector<int>(n));
    for(int a_i = 0;a_i < n;a_i++){
       for(int a_j = 0;a_j < n;a_j++){
          cin >> a[a_i][a_j];
       }
    }
    
    int sum1=0,sum2=0;   

for(int i=0, k=n-1; i< n; i++, k--){
    for(int j=0; j< n; j++){
        cin >> a[i][j];            
        if(i==j)
         sum1 += a[i][j];                        
    }
    sum2 += a[i][k];              
}

cout << abs(sum1-sum2);
   
    return 0;
}