#include <bits/stdc++.h>
using namespace std;
using lint     = long long;
const lint inf = 1LL << 60;
const lint mod = 1000000007;

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    lint n, k;
    cin >> n >> k;
    k--;
    vector<int> a(n);
    for (int i = 0; i < n; ++i) {
        cin >> a[i];
    }
    lint ret = a[k];
    if (a[k] == 0) {
        cout << 0 << "\n";
        return 0;
    }
    lint retr = 0;
    for (int i = k + 1; i < n; ++i) {
        retr += a[i];
        if (a[i] == 1 || a[i] == 0)
            break;
    }
    lint retl = 0;
    for (int i = k - 1; i >= 0; --i) {
        retl += a[i];
        if (a[i] == 1 || a[i] == 0)
            break;
    }
    if (a[k] == 1) {
        ret += max(retl, retr);
    } else {
        ret += retl + retr;
    }
    cout << ret << "\n";
    return 0;
}