import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.LinkedList;
import java.util.Queue;

public class Main {

  static boolean[] visited;
  static int[] map;
  static int cnt = 1;

  public static void bfs(int start) {
    Queue<Integer> queue = new LinkedList<>();
    queue.add(start);
    while (!queue.isEmpty()) {
      int poll = queue.poll();
      visited[poll] = true;

      if (poll + map[poll] < map.length) {
        if (!visited[poll + map[poll]]) {
          queue.add(poll + map[poll]);
          visited[poll + map[poll]] = true;
          cnt++;
        }
      }

      if (poll - map[poll] > -1) {
        if (!visited[poll - map[poll]]) {
          queue.add(poll - map[poll]);
          visited[poll - map[poll]] = true;
          cnt++;
        }
      }

    }
  }

  public static void main(String[] args) throws IOException {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    int count = Integer.parseInt(br.readLine());
    map = Arrays.stream(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();
    visited = new boolean[count];
    bfs(Integer.parseInt(br.readLine()) - 1);
    System.out.println(cnt);
  }
}