Here is the code the 5th period wrote today that isn't in the slides:

public int sum13(int[] nums) {
  int sub = 0;
  int sum = 0;
  for(int i=0; i < nums.length; i++) {
    if(nums[i] == 13)
      sub += (nums[i] + nums[i+1]);
    sum += nums[i];
  }
  return sum - sub;
}


public int sum13(int[] nums) {
  int sub = 0;
  int sum = 0;
  for(int i=0; i < nums.length; i++) {
    if(nums[i] == 13) {
      if(i < nums.length-1 && nums[i+1] != 13) {
        sub += (nums[i] + nums[i+1]);
      } else {
        sub += nums[i];
      }
    }
    sum += nums[i];
  }
  return sum - sub;
}