Life Changing System1

Aerya City was a big city with 1 million population. However, the city was the only place safe for human to live because everything around the city was dangerous. Lein was a 15 years old boy. A monster wave hit Aerya City and some of the monsters managed to get inside the city. Lein was unlucky and got killed by one of the monsters. However, a miracle happened and Lein came back to life a day after he died. Lein entered the academy and awakened his power. After awakening his power, Lein gained a system called Life Exchange System. He can use the system to get stronger but with his lifespan as the payment.