#include <bits/stdc++.h>

using namespace std;

using ll = long long;

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n, K;
    cin >> n >> K;
    --K;
    vector<ll> a(n);
    for (int i = 0; i < n; ++i) {
        cin >> a[i];
    }

    if (a[K] == 0) {
        cout << 0 << "\n";
        return 0;
    }

    ll cnt_r = 1;
    for (int i = K + 1; i < n; ++i) {
        cnt_r += a[i];
        if (a[i] == 1) break;
    }

    ll cnt_l = 1;
    for (int i = K - 1; i >= 0; --i) {
        cnt_l += a[i];
        if (a[i] == 1) break;
    }

    if (a[K] == 1) {
        cout << max(cnt_l, cnt_r) << "\n";
    } else {
        cout << cnt_l + cnt_r + a[K] - 2 << "\n";
    }
    return 0;
}