diagonal_difference.js

function main() {
    var n = parseInt(readLine());
    var a = [];
    for(a_i = 0; a_i < n; a_i++){
       a[a_i] = readLine().split(' ');
       a[a_i] = a[a_i].map(Number);
    }
    
    let sumPrimaryDiagonal = sumSecondaryDiagonal = 0;
    
    for (i = 0, len = a.length; i < len; i++) {
        let line = a[i];
        sumPrimaryDiagonal += line[i];
        sumSecondaryDiagonal += line[(len -1) - i];
    }
    console.log(Math.abs(sumPrimaryDiagonal - sumSecondaryDiagonal))

}