#include <bits/stdc++.h>
using namespace std;
using lint = long long;
using pint = pair<int, int>;
using vi = vector<int>;
#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define all(v) v.begin(), v.end()
#define endl "\n"

constexpr int MOD = 1000000007;
const int INF = 1 << 30;

int main() {
    int n, k;
    cin >> n >> k;
    k--;
    vi a(n);
    rep(i, n) cin >> a[i];

    lint res = a[k];
    if (a[k] == 0) {
        cout << 0 << endl;
        exit(0);
    }

    lint l = 0, r = 0;

    for (int i = k-1; i >= 0; i--) {
        l += a[i];
        if (a[i] <= 1) break;
    }

    for (int i = k+1; i < n; i++) {
        r += a[i];
        if (a[i] <= 1) break;
    }

    if (a[k] == 1) res += max(l, r);
    else {
        res += l;
        res += r;
    }

    cout << res << endl;
    return 0;
}