#include <bits/stdc++.h>
using namespace std;

int main() {
    int n, k;
    cin >> n >> k;
    k--;
    vector<int> a(n);
    for (int i = 0; i < n; i++) cin >> a.at(i);
    int64_t ans = a.at(k);
    if (a.at(k) == 0) {
        cout << 0 << endl;
        return 0;
    }
    int64_t ansr = 0, ansl = 0;
    for (int i = k + 1; i < n; i++) {
        ansr += a.at(i);
        if (a.at(i) <= 1) break;
    }
    for (int i = k - 1; i >= 0; i--) {
        ansl += a.at(i);
        if (a.at(i) <= 1) break;
    }
    if (ans == 1) {
        ans += max(ansl, ansr);
    } else {
        ans += ansl + ansr;
    }
    cout << ans << endl;
}