#include<stdio.h>
int main()
{
	int n, k;
	scanf("%d %d", &n, &k);
	int i;
	long long int a[200005];
	for (i = 0; i < n; i++)
		scanf("%lld", &a[i]);
	long long int ans[2];
	k--;
	if (a[k] == 0)
	{
		printf("0\n");
		return 0;
	}
	ans[0] = 0;
	for (i = k - 1; i >= 0; i--)
	{
		ans[0] += a[i];
		if (a[i] < 2)
			break;
	}
	ans[1] = 0;
	for (i = k + 1; i < n; i++)
	{
		ans[1] += a[i];
		if (a[i] < 2)
			break;
	}
	if (a[k] == 1)
	{
		if (ans[0] > ans[1])
			printf("%lld\n", ans[0] + 1);
		else
			printf("%lld\n", ans[1] + 1);
	}
	else
		printf("%lld\n", ans[0] + ans[1] + a[k]);
	return 0;
}