Context: Ryu (Ha-kyun Shin), a deaf man, works in a factory to support his ailing sister (Ji-Eun Lim) in desperate need of a kidney transplant. Ryu tries to donate one of his kidneys to his sister but is told that his blood type does not match that of his sister and so is not a suitable donor. After being laid off from his job by the factory boss, Ryu contacts a black market organ dealer who agrees to sell him a kidney suitable for his sister in exchange for 10,000,000 Korean won, plus one of Ryu's own kidneys. He takes the severance pay from his factory job and offers the money to the organ dealers, who take the money and one of his kidneys, but then disappear. Three weeks later, Ryu learns from his doctor that a kidney has been found for his sister and that the operation will cost 10,000,000 won, but since the organ dealers stole his money, he will not be able to pay for it.In need of money for the operation and in retaliation for his being fired, Ryu and his girlfriend, Yeong-mi (Doona Bae), a radical anarchist, conspire to kidnap the daughter of the boss who fired him. Instead, he realizes that the kidnapping would immediately put them under police suspicion, and they decide to kidnap Yu-sun (Bo-bae Han), the daughter of the boss's friend, Park Dong-jin (Kang-ho Song), another factory executive. The girl stays with Ryu's sister (thinking that Ryu is merely babysitting for a time), who takes care of her while the distraught Dong-jin arranges to pay her ransom. After Ryu collects the money and returns home, he learns that his sister has discovered his scheme and, unwilling to be involved or burden Ryu further, killed herself. Ryu takes Yu-sun and his sister's body into the countryside to bury her by a riverbed they used to frequent as children. While Ryu mourns, Yu-sun accidentally slips into the river and drowns.Days later, as Dong-jin mourns his daughter and swears revenge at the river bank, Ryu ambushes and murders the organ dealers. Dong-jin, having investigated the identities of the kidnappers, finds Yeong-mi and begins interrogating her. Yeong-mi apologizes for Yu-sun's death but warns him of her membership in a terrorist organization that, knowing Dong-jin's identity, will kill him if she dies. Dong-jin, unfazed by the threats, tortures her to death by electrocuting her. Ryu returns to Yeong-mi's apartment building and discovers the police removing her body on a stretcher. Ryu, consumed with grief, swears vengeance on Dong-jin.Ryu arrives at Dong-jin's residence in an attempt to kill him. He waits for some time, but Dong-Jin does not arrive: he is, in fact, waiting at Ryu's apartment. After Dong-Jin does not arrive, Ryu returns to his apartment. However, Dong-jin had previously set up an electric booby trap on his doorknob, which renders Ryu unconscious. Dong-jin then binds Ryu and returns him to the riverbed where Yu-sun died. After binding Ryu's hands and feet and bringing him chest-high into the water, an emotional Dong-jin acknowledges that although Ryu is a good man, he has no choice; Dong-jin then slashes Ryu's Achilles tendons, resulting in his drowning.Dong-jin drags Ryu back to shore and then drives off to a desolate location to bury the body. Once there, he begins to dig a hole, but soon a group of men arrives. They surround and stab Dong-jin repeatedly, finally attaching a note to his chest identifying themselves as the terrorist group of which Yeong-mi was part. The group leave Dong-jin dying beside his car with the bloody tools and bags he used to chop up, dismember, and package Ryu's body.(From Wikipedia.) 
 Question: How does Yu-sun die?
Output: drowns