CF407B.Long Path

One day, little Vasya found himself in a maze consisting of (n + 1) rooms, numbered from 1 to (n + 1). Initially, Vasya is at the first room and to get out of the maze, he needs to get to the (n + 1)-th one.

The maze is organized as follows. Each room of the maze has two one-way portals. Let's consider room number i (1 ≤ i ≤ n), someone can use the first portal to move from it to room number (i + 1), also someone can use the second portal to move from it to room number p__i, where 1 ≤ p__i ≤ i.

In order not to get lost, Vasya decided to act as follows.

• Each time Vasya enters some room, he paints a cross on its ceiling. Initially, Vasya paints a cross at the ceiling of room 1.
• Let's assume that Vasya is in room i and has already painted a cross on its ceiling. Then, if the ceiling now contains an odd number of crosses, Vasya uses the second portal (it leads to room p__i), otherwise Vasya uses the first portal.

Help Vasya determine the number of times he needs to use portals to get to room (n + 1) in the end.

一天，小瓦夏发现自己身处一个由 $n+1$ 个房间组成的迷宫中，房间编号从 $1$ 到 $n+1$。初始时，瓦夏位于第 $1$ 个房间，而要走出迷宫，他需要到达第 $n+1$ 个房间。

迷宫的结构如下：迷宫中的每个房间都有两个单向传送门。考虑编号为 $i$ 的房间（$1 \le i \le n$），可通过第一个传送门从该房间移动到编号为 $i+1$ 的房间；也可通过第二个传送门从该房间移动到编号为 $p_i$ 的房间，其中 $1 \le p_i \le i$。

为了不迷路，瓦夏决定按如下方式行动：

• 每次瓦夏进入某个房间时，他都会在该房间天花板上画一个叉号。初始时，瓦夏在第 $1$ 个房间的天花板上画了一个叉号。
• 假设瓦夏当前位于房间 $i$，且已在该房间天花板上画过一个叉号。那么，若此时天花板上的叉号总数为奇数，瓦夏就使用第二个传送门（通向房间 $p_i$）；否则，瓦夏使用第一个传送门。

请帮助瓦夏计算：他最终到达房间 $n+1$ 所需使用的传送门总次数。

The first line contains integer n (1 ≤ n ≤ 103) — the number of rooms. The second line contains n integers p__i (1 ≤ p__i ≤ i). Each p__i denotes the number of the room, that someone can reach, if he will use the second portal in the i-th room.

第一行包含一个整数 $n$（$1 \leq n \leq 10^3$）—— 房间的数量。
第二行包含 $n$ 个整数 $p_i$（$1 \leq p_i \leq i$）。每个 $p_i$ 表示：若某人在第 $i$ 个房间中使用第二个传送门，则他所能到达的房间编号。

Print a single number — the number of portal moves the boy needs to go out of the maze. As the number can be rather large, print it modulo 1000000007 (109 + 7).

输出一个整数——男孩逃出迷宫所需的传送门移动次数。由于该数可能非常大，请对 $1000000007$（即 $10^9 + 7$）取模后输出。