text: /**
   * Big integer add operation.
   * @param a {String}
   * @param b {String}
   * @returns {String}
   */
  const add = (a, b) => {
      let i = a.length -1,
          j = b.length - 1,
          carry = 0,
          ret = '';
  
      while (i >= 0 || j >= 0) {
          let x = 0, y = 0, sum;
  
          if (i >= 0) {
              x = a[i] - '0';
              i --;
          }
  
          if (j >= 0) {
              y = b[j] - '0';
              j --;
          }
  
          sum = x + y + carry;
          if (sum >= 10) {
              carry = 1;
              sum -= 10;
          }
  
          ret = sum + ret;
      }
  
      if (carry) {
          ret = carry + ret;
      }
  
  
      return ret;
  };