easy approach

public static int viralAdvertising(int n) {
    if (n <= 0) return -1;
    int likes = 0;
    int peopleShare = 5; 
    for (int i = 1; i <= n; i++) {
        int peopleLike = (int) Math.floor(peopleShare / 2); // 2
        likes += peopleLike;
        peopleShare = peopleLike * 3;
    }
    return likes;
}