from bisect import bisect_left


def main():
    length, start = map(int, input().split())
    start -= 1
    coins = [int(x) for x in input().split()]
    only_1_coin_index = []
    for i in range(length):
        if i == start:
            continue
        if coins[i] == 1:
            only_1_coin_index.append(i)
    answer = 0
    if coins[start] > 1:
        nearest_1_coin = bisect_left(only_1_coin_index, start)
        if nearest_1_coin == 0:
            answer = max(sum(coins[:start + 1]), sum(coins[start: only_1_coin_index[0] + 1]))
        elif nearest_1_coin == len(only_1_coin_index):
            answer = max(sum(coins[only_1_coin_index[0]:start + 1]), sum(coins[start:]))
        else:
            left = only_1_coin_index[nearest_1_coin - 1]
            right = only_1_coin_index[nearest_1_coin]
            answer = sum(coins[left:right + 1])
    print(answer)


if __name__ == '__main__':
    main()