## https://yukicoder.me/problems/no/1868

import heapq

MAX_INT = 10 ** 18

def main():
    N = int(input())
    R = list(map(int, input().split()))


    dp = [MAX_INT] * N

    queue = []
    heapq.heappush(queue,(0, 0))

    for x in range(N):

        while len(queue) > 0 and queue[0][-1] < x:
            heapq.heappop(queue)
        
        v = queue[0][0]
        dp[x] = v

        if x < N - 1:
            heapq.heappush(queue, (v + 1, R[x] - 1))

    print(dp[-1])



    




if __name__ == "__main__":
    main()