B"There are n startups. Startups can be active or acquired. If a startup is acquired, then that means it has exactly one active startup that it is following. An active startup can have arbitrarily many acquired startups that are following it. An active startup cannot follow any other startup. The following steps happen until there is exactly one active startup. The following sequence of steps takes exactly 1 day. For example, the following scenario can happen: Let's say A , B are active startups. C , D , E are acquired startups under A , and F , G are acquired startups under B : Active startups are shown in red. If A acquires B , then the state will be A , F , G are active startups. C , D , E , B are acquired startups under A . F and G have no acquired startups: If instead, B acquires A , then the state will be B , C , D , E are active startups. F , G , A are acquired startups under B . C , D , E have no acquired startups: You are given the initial state of the startups. For each startup, you are told if it is either acquired or active. If it is acquired, you are also given the index of the active startup that it is following. You're now wondering, what is the expected number of days needed for this process to finish with exactly one active startup at the end. It can be shown the expected number of days can be written as a rational number P/Q , where P and Q are co-prime integers, and Q not= 0 pmod{10^9+7} . Return the value of P cdot Q^{-1} modulo 10^9+7 . The first line contains a single integer n ( 2 <= q n <= q 500 ), the number of startups. The next line will contain n space-separated integers a_1, a_2, ldots, a_n ( a_i = -1 or 1 <= q a_i <= q n ). If a_i = -1 , then that means startup i i"...