貝殼w

'''在步長(zhǎng)限制范圍內(nèi)，通過(guò)迭代更新當(dāng)前的索引及下一步的步長(zhǎng)'''
#num_arr = input().split(' ')
#num_arr = [int(i) for i in num_arr]
num_arr = [7,5,9,4,2,6,8,3,5,4,3,9] # 示例 ，輸出為2
N = len(num_arr)
if N%2==0:
    max_step_length = N/2-1
else:
    max_step_length = N/2
num_list = []
for step in range(1,int(max_step_length)):
    index_next = step
    num = 1
    while index_next<N:
        next_step = num_arr[index_next]
        index_next = index_next + next_step
        if index_next<N:
            num+=1
        if index_next == N-1:
            num_list.append(num)
            # print("step:",step)
            # print("num_list:",num_list)
        if index_next>=N:
            num_list.append(-1)
out_num = [ i for i in num_list if i>0]
if len(out_num)==0:
    print(-1)
else:
    print(min(out_num))