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 int[] map;
  static boolean[] visited;
  static int ans = -1;

  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[map.length];
    bfs(0);
    System.out.println(ans);
  }

  public static void bfs(int start) {
    Queue<int[]> queue = new LinkedList<>();
    queue.add(new int[]{start, 0});
    while (!queue.isEmpty()) {
      int[] poll = queue.poll();
      visited[poll[0]] = true;
      if (poll[0] == map.length - 1) {
        if (ans == -1) {
          ans = poll[1];
        } else {
          ans = Math.min(ans, poll[1]);
        }
        break;
      }
      for (int i = 1; i <= map[poll[0]]; i++) {
        if (poll[0] + i < map.length) {
          if (!visited[poll[0] + i]) {
            queue.add(new int[]{poll[0] + i, poll[1] + 1});
            visited[poll[0] + i] = true;
          }
        }
      }
    }
  }
}