Taxpayer X filed a 2021 return on February 21, 2022, showing a tax liability of $8,700. On August 15, 2022, he filed an amended return showing an additional tax liability of $737 (he found a couple of Forms 1099 he had missed when he prepared and filed his original return). Later, in October 2022, he received a letter from a Service Center (as a result of additional information returns he had missed) proposing an additional tax liability of $125 (for a total correct tax liability for 2021 of $9,562). By definition, how much is the tax deficiency