N = int(input())
L = list(map(int,input().split()))
D = [99999999]*(N+200)
D[0] = 0
for i in range(N):
for j in range(1,L[i]+1):
if D[i+j] > D[i] + 1:
D[i+j] = D[i]+1
if D[N-1] == 99999999:
print(-1)
else:
print(D[N-1])