Mary is John's surviving spouse. He used $450,000 of his gift and estate tax exclusion for his

Question:

Mary is John's surviving spouse. He used $450,000 of his gift and estate tax exclusion for his lifetime gifts and his estate used $2,300,000 of his estate tax exclusion in closing his estate. What is Mary's estate tax exclusion when she dies if she and John elected to gift split on all taxable gifts?