"""Day 15.

The 30000000 takes a while to complete
"""


with open("input") as f:
    memory = list(map(int, f.read().rstrip().split(",")))

data = {val: {"position": pos + 1, "count": 1} for pos, val in enumerate(memory)}

pos = len(memory) + 1
last = memory[-1]

# for x in range(2020 - pos + 1):
for x in range(30000000 - pos + 1):
    if data[last]["count"] == 1:
        val = 0
    else:
        x = pos - 1
        y = data[last]["position"]
        val = x - y
        data[last]["position"] = x
        if val not in data:
            data[val] = {"position": pos, "count": 1}
    last = val
    data[val]["count"] += 1
    pos += 1

print(val)