#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); i++)
#define ALL(v) (v).begin(), (v).end()
using ll = long long;
using P = pair<int, int>;
const int INF = 1e9;
const long long LINF = 1e18;
const long long MOD = 1e9 + 7;

signed main() {
    int n, k;
    cin >> n >> k;
    k--;
    ll a[n];
    rep(i, n) {
        cin >> a[i];
    }
    ll ans = 0;
    if (a[k] == 0) {
        cout << 0 << endl;
        return 0;
    }
    ll right = 0, left = 0;
    for (int i = k + 1; i < n; i++) {
        right += a[i];
        if (a[i] == 0 || a[i] == 1) break;
    }
    for (int i = k - 1; i >= 0; i--) {
        left += a[i];
        if (a[i] == 0 || a[i] == 1) break;
    }
    if (a[k] == 1) {
        cout << max(left, right) + 1 << endl;
        return 0;
    }
    cout << left + right + a[k] << endl;
    return 0;
}