"""Day 15."""


# reading off numbers
# starting numbers in input
# consider the most recently spoken number

# new unique numbers mean the current says zero
# how far aprat the previous spoken one was


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

last_pos = {val: pos + 1 for pos, val in enumerate(memory)}

pos = len(memory) + 1
for x in range(2020 - pos + 1):
    last = memory[-1]
    if memory.count(last) == 1:
        memory.append(0)
    else:
        x = pos - 1
        y = last_pos[last]
        val = x - y
        memory.append(val)
        last_pos[last] = x
        if val not in last_pos:
            last_pos[val] = pos
    pos += 1

print(memory[-1])