If the neighbors do not want to voluntarily compensate you for the material damage caused to you in connection with the flooding of the apartment, then you need to go to court.
As a general rule, a lawsuit against a natural person is filed in court at the place of residence or stay registered in accordance with the procedure established by law, unless otherwise provided by law. Also, a claim for compensation for damage caused to the property of individuals or legal entities may be filed at the place of damage.
The first thing to do is to record the fact that the apartment is flooded. To do this, you need to contact the balance keeper (JEK or condominium). After that, the relevant commission will draw up a report on the flooding of the apartment, which must indicate the nature of the flooding, its causes, the damage caused, a list of damaged items, and the amount of necessary repairs. The appeal to the balance keeper should be submitted in writing and keep the second copy with you. I also advise you to record the damage caused by the flooding of the apartment in a photo or video. The above-mentioned evidence will be extremely important when the court considers the case.
Also, the court must confirm the amount of the damages incurred. To do this, you need to contact the relevant expert institution with a request to conduct a construction and technical expert study, based on the results of which you can determine the causes and extent of the damage caused by the flooding of the apartment. The expert will provide a report on the determination of the cost of restorative repair and construction works that must be carried out, as well as a conclusion on the valuation of the property, which can be attached to the case in the future.