why not pass all test case in problem balanced numbers in codewars?

Currently i am doing a challenge in codewars on Balanced numbers and i wrote the code in dart and it successfully completed 100 test cases but for long numbers it is not working properly...So i think that it need some conditions for this long numbers:

String balancedNum(numb) {
  // your code here
  var numb1 = numb.toString();
  List a = numb1.split("");

  var left_sum = 0;
  var right_sum = 0;

  for (var i = 0; i <= a.length / 2; i++) {
    left_sum += int.parse(a[i]);
  }

  List<String> b = a.reversed.toList();

  //print(b);

  for (var i = 0; i < b.length / 2; i++) {
    right_sum += int.parse(b[i]);
  }

  //print(right_sum);

  if (left_sum == right_sum) {
    return 'Balanced';
  } else {
    return 'Not Balanced';
  }
}

link to the challenge:https://www.codewars.com/kata/balanced-number-special-numbers-series-number-1/train/dart

The compiler verdict for the code is wrong answer because of a simple logical error.

This is because the length of the list has not been calculated correctly

Kindly have a look at the corrected code below, which passes all the 110 test on the platform:

String balancedNum(numb) {
      // your code here
      var numb1 = numb.toString();
      List a = numb1.split("");

      var left_sum = 0;
      var right_sum = 0;

      double d = ((a.length-1) / 2);
      int len = d.floor();

      print(len);

      for (var i = 0; i < len; i++) {
            left_sum += int.parse(a[i]);
      }

      List<String> b = a.reversed.toList();

      print(b);

      for (var i = 0; i < len; i++) {
            right_sum += int.parse(b[i]);
      }

      print(left_sum);
      print(right_sum);

      if (left_sum == right_sum) {
                return 'Balanced';
      } else {
                return 'Not Balanced';
      }
}