Lydia and John Wickham filed jointly in Year $1.$ They divorced in Year $2.$ Late in Year $2,$ the IRS discovered that the Wickhams had underpaid their Year $1$ taxes by $$2,000.$ Both Lydia and John worked in Year $1$ and received equal income but John had $$2,000$ less tax withheld than Lydia did. Who is legally liable for the tax underpayment?