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 = coins[start]
    if coins[start] > 1:
        move_range = [0, 0]
        only_1_coin_index.sort()
        if only_1_coin_index[-1] < start:
            answer = sum(coins[only_1_coin_index[-1]:])
        elif start < only_1_coin_index[0]:
            answer = sum(coins[:only_1_coin_index[0] + 1])
        else:
            for i in range(len(only_1_coin_index) - 1):
                if only_1_coin_index[i] < start < only_1_coin_index[i + 1]:
                    move_range[0] = only_1_coin_index[i]
                    move_range[1] = only_1_coin_index[i + 1]
                    break
            answer = sum(coins[move_range[0]:move_range[1] + 1])
    print(answer)


if __name__ == '__main__':
    main()