Plant Trees, Save Life

There are n students in a school. Some of them are friends with each other. You want to make the school clean. So you have to give ice-cream to each student so he or she starts planting trees. When one plants a tree, he/she tells it to all of his friends and they start to plant trees and tell their friends too (without any ice creams) and so on.

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why the ans cant be submitted, it is showing unauthorized.