It is expressly agreed and understood that in the event production of oil, gas or other minerals is obtained from the herein leased premises, and the aggregate royalties paid to Lessor in the twelve (12) month period following the date of first production, or in any succeeding twelve (12) month period amounts to less than ________________ and No/100 Dollars ($_______.00) per net mineral acre per annum for each such acre subject to this lease during any such twelve (12) month period, then, and in that event, the Lessee, its successors or assigns, agrees to pay to Lessor [within ninety (90) days after the expiration of any such twelve (12) month period by depositing same to Lessor's credit in the depository hereinabove named] the difference, if any, between the total amount of royalties and shut-in royalties received by Lessor under this lease during such twelve (12) month period and the aggregate of ________________ and No/100 Dollars ($______.00) per net mineral acre for the acreage subject to this lease during such twelve (12) month period. For the purposes of this paragraph, the "date of first production" shall mean the first day of the next calendar month following the day upon which oil, gas or other minerals are first produced and sold from the leased premises or acreage, and each "succeeding twelve (12) month period" shall commence on the anniversary date of the date of the "date of first production."