#include <cstdio>
#include <iostream>
#include <string>
#include <sstream>
#include <stack>
#include <algorithm>
#include <cmath>
#include <queue>
#include <map>
#include <set>
#include <cstdlib>
#include <bitset>
#include <tuple>
#include <assert.h>
#include <deque>
#include <bitset>
#include <iomanip>
#include <limits>
#include <chrono>
#include <random>
#include <array>
#include <unordered_map>
#include <functional>
#include <complex>

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

constexpr long long MAX = 5100000;
constexpr long long INF = 1LL << 60;
constexpr int inf = 1 << 28;
constexpr long long mod = 1000000007LL;
//constexpr long long mod = 998244353LL;

using namespace std;
typedef unsigned long long ull;
typedef long long ll;


int main()
{
	/*
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	*/
	int n, k; scanf("%d %d", &n, &k);
	vector<ll> a(n); for (int i = 0; i < n; i++) scanf("%lld", &a[i]);
	int s = k - 1;
	if (a[s] == 0) {
		puts("0");
		return 0;
	}
	if (a[s] == 1) {
		ll left = 0, right = 0;
		for (int i = s - 1; i >= 0; i--) {
			if (a[i] == 0) break;
			if (a[i] == 1) {
				left++;
				break;
			}
			left += a[i];
		}
		for (int i = s + 1; i < n; i++) {
			if (a[i] == 0) break;
			if (a[i] == 1) {
				right++;
				break;
			}
			right += a[i];
		}
		printf("%lld\n", a[s] + max(left, right));
	}
	else {
		ll left = 0, right = 0;
		for (int i = s - 1; i >= 0; i--) {
			if (a[i] == 0) break;
			if (a[i] == 1) {
				left++;
				break;
			}
			left += a[i];
		}
		for (int i = s + 1; i < n; i++) {
			if (a[i] == 0) break;
			if (a[i] == 1) {
				right++;
				break;
			}
			right += a[i];
		}
		printf("%lld\n", a[s] + left + right);
	}
	return 0;
	/*
		おまじないを使ったらscanfとprintf関連注意！！！！！！！！！！！！
	*/
}