#include <algorithm>
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define all(x) (x).begin(), (x).end()
#define ll long long
#define ld long double
#define INF 1000000000000000000
typedef pair<ll, ll> pll;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    int N, K;
    cin >> N >> K;
    K--;
    vector<ll> A(N);
    rep(i, N) { cin >> A[i]; }

    bool judge = (A[K] >= 3 ? 1 : 0);

    // right
    ll right = K, rtmp = 0;
    while (right < N) {
        if (A[right] >= 2)
            rtmp += A[right];
        else if (A[right] == 1) {
            rtmp += A[right];
            break;
        } else
            break;
        right++;
    }

    // left
    ll left = K, ltmp = 0;
    while (left > 0) {
        if (A[left] >= 2)
            ltmp += A[left];
        else if (A[left] == 1) {
            ltmp += A[left];
            break;
        } else
            break;
        left--;
    }

    if (judge && rtmp > 0 && ltmp > 0) {
        cout << rtmp + ltmp - A[K] << endl;
    } else if (judge) {
        cout << rtmp + ltmp << endl;
    } else
        cout << max(rtmp, ltmp) << endl;
}