n = int(input())
if n == 1:
    print(0)
    exit()
R = list(map(int, input().split()))

jumps = 0
current_start = 1
current_end = 1

while current_end < n:
    max_i = min(current_end, n-1)
    if current_start > max_i:
        print(-1)
        exit()
    next_max = 0
    for i in range(current_start, max_i + 1):
        next_max = max(next_max, R[i-1])
    jumps += 1
    current_start = current_end + 1
    current_end = next_max

print(jumps)