#include <stdio.h>
#include <stdlib.h>

int calc_earnable_coin(int length, int* list, int start, int step);

int main(void) {
    int point_num, position;
    int *coin_count;
    int result;
    
    scanf("%d %d", &point_num, &position);
    position--;
    
    coin_count = (int*) malloc(sizeof(int)*point_num);
    
    for(int i=0; i<point_num; i++) {
        scanf("%d", &coin_count[i]);
        //printf("%d ", coin_count[i]);
    }
    
    if(coin_count[position] == 0) {
        result = 0;
    } else {
        int count_left = 0, count_right = 0;
        count_left = calc_earnable_coin(point_num, coin_count, position-1, -1);
        count_right = calc_earnable_coin(point_num, coin_count, position+1, 1);
        
        if(coin_count[position] == 1) {
            if(count_left >= count_right) {
                result = count_left + coin_count[position];
            } else {
                result = count_right + coin_count[position];
            }
        } else {
            result = count_left + count_right + coin_count[position];
        }
    }
    
    free(coin_count);
    printf("%d\n", result);
    return 0;
}

int calc_earnable_coin(int length, int* list, int start, int step) {
    int count=0;
    if(start >=  0 || start < length) {
        for(int i=start; i!=0 && i!=length; i+=step) {
            count += list[i];
            if(list[i] < 2) {
                break;
            }
        }
    }
    return count;
}