from collections import deque
N = int(input())
A = list(map(int, input().split()))
d = deque()
seen = [False]*N
dis = [-1]*N
dis[0] = 0
d.append(0)
while d:
    v = d.popleft()
    if v == N-1:
        break
    for i in range(v+1, A[v]):
        if not seen[i]:
            dis[i] = dis[v]+1
            seen[i] = True
            d.append(i)
print(dis[-1])