题解 | #跳跃游戏(三)# Python3

import sys


# 下一个要跳跃的点，是在当前能跳跃的点里面，能跳最远的
# 因为其他点的话，当前能跳跃的点能到达其他所有点能跳跃的地方

n = int(input())


if n==0:
    print(-1)
elif n == 1:
    print(0)
else:
    nums = list(map(int,input().strip().split(' ')))
    boundary = nums[0]
    max_len = nums[0]
    time = 0
    next_boundary = nums[0]
    for i in range(1,n):
        next_boundary = max(nums[i]+i, next_boundary)
        next_boundary = min(next_boundary, n-1)
        if i == boundary:
            boundary = next_boundary
            time += 1
    if boundary < n-1:
        print(-1)
    else:
        print(time)