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from numba import jit

@jit
def run(cmds):
    location = 0
    counter = 0
    l = len(cmds)
    while 1:
        cmd = cmds[location]
        if cmd >= 3:
            cmds[location] -= 1
        else:
            cmds[location] += 1
        location += cmd
        if location < 0 or location > l:
            print(counter)
            break
        counter += 1

if __name__=="__main__":
    text = open("input.txt").read().strip().split("\n")
    cmds = [int(cmd) for cmd in text]
    run(cmds)