The floor slab sags about 4 - 5 cm

I am interested in buying a 30-year-old single-family house. The ceiling joist sags about 4 - 5 cm over a distance of 4 m. On the bottom, there is drywall, and on top, wooden subfloor panels are installed. What is the most cost-effective: removing the drywall from below and reinforcing the beams or opening the subfloor panels from above?

You should have that checked by a professional. Because if the ceiling sags that much, I find that very concerning! What if you have heavy furniture or even a waterbed, then you can expect the ceiling to break through.