Integrate two HTML page into one HTML page without affecting the design of the header and footer

Integrate a page without header and footer, into a page with header and footer without affecting the design of the header and footer.

Integrate this calculator:
https://ladcalculation.com/ladcalculatorfirstgeneration.html

Into This Page
https://ladcalculation.com/ladcalculator.html

*Do not affect the design of header and footer.