#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using VI = vector<int>;
using VL = vector<ll>;
using PII = std::pair<int, int>;
using PLL = std::pair<ll, ll>;

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repr(i, n) for (int i = (int)(n)-1; i >= 0; i--)
#define rep2(i, s, n) for (int i = (s); i < (int)(n); i++)
#define allpt(v) (v).begin(), (v).end()
#define allpt_r(v) (v).rbegin(), (v).rend()



const int mod = 1e9 + 7;
const string wsp = " ";
const string tb = "\t";
const string rt = "\n";

template <typename T>
void show1dvec(vector<T> v)
{
    int n = v.size() - 1;
    rep(i, n) cout << v[i] << wsp;
    cout << v[n] << rt;
    return;
}

template <typename T>
void show2dvec(vector<vector<T>> v)
{
    int n = v.size();
    rep(i, n) show1dvec(v[i]);
}

int main()
{

#ifdef DEBUG
    cout << "DEBUG MODE" << endl;
    ifstream in("input.txt"); //for debug
    cin.rdbuf(in.rdbuf());    //for debug
#endif

    int n, k, l1, r1;
    ll ans = 0, s = 0;
    cin >> n >> k;
    VL candies(n + 1, 0), cum_candies(n + 1, 0);

    rep(i, n)
    {
        cin >> candies[i + 1];
        s += candies[i + 1];
        cum_candies[i + 1] = s;
    }

    l1 = r1 = k;
    rep2(i, k + 1, n + 1)
    {
        if (candies[i] <= 1)
        {
            r1 = i;
            break;
        }

    }

    for (int i = k - 1; i > 0; i--)
    {

        if (candies[i] <= 1)
        {
            l1 = i;
            break;
        }
    }

    if (candies[k] == 0)
        ans = 0;
    
    else if (candies[k] == 1)
    {
        if (l1 == k && r1 == k) ans = max(cum_candies[n] - cum_candies[k - 1], cum_candies[k]);
        else if (l1 == k) ans = max(cum_candies[r1] - cum_candies[k - 1], cum_candies[k]);
        else if (r1 == k) ans = max(cum_candies[n] - cum_candies[k - 1], cum_candies[k] - cum_candies[l1 - 1]);
        else ans = max(cum_candies[r1] - cum_candies[k - 1], cum_candies[k] - cum_candies[l1 - 1]);
    }

    else if (candies[k] >= 2)
    {
        if (l1 == k && r1 == k) ans = cum_candies[n];
        else if (l1 == k) ans = cum_candies[r1];
        else if (r1 == k) ans = cum_candies[n] - cum_candies[l1 - 1];
        else ans = cum_candies[r1] - cum_candies[l1 - 1];
    }

    cout << ans << rt;

    return 0;
}