d5.py

def parse(s):
    n = []
    for l in s.split('\n'):
        if l != '':
            n.append(int(l))
    return n

with open('d5in.txt') as f:
    nums = parse(f.read())
pos = 0
c = 0

while pos < len(nums) and pos >= 0:
    n = nums[pos]
    if n >= 3:
        nums[pos] -= 1
    else:
        nums[pos] += 1
    pos += n
    c += 1

print(c)