#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <cmath>
#include <stdio.h>
#include <queue>
#include <deque>
#include <cstdio>
#include <set>
#include <map>
#include <bitset>
#include <stack>
#include <cctype>
using namespace std;
long long a[200020];
int main() {
	int n, k;
	cin >> n >> k;
	k--;
	for (int i = 0; i < n; i++) {
		cin >> a[i];
	}
	if (a[k] == 0) {
		cout << "0" << endl;
		return 0;
	}
	long long l = 0, r = 0;
	for (int i = k + 1; i < n; i++) {
		r += a[i];
		if (a[i] < 2) {
			break;
		}
	}
	for (int i = k - 1; i >= 0; i--) {
		l += a[i];
		if (a[i] < 2) {
			break;
		}
	}
	if (a[k] == 1) {
		cout << max(l, r) + 1 << endl;
	}
	else {
		cout << l + r + a[k] << endl;
	}
}