#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for(ll i = 0, i##_len = (n); i < i##_len; i++)
#define reps(i, s, n) for(ll i = (s), i##_len = (n); i < i##_len; i++)
#define rrep(i, n) for(ll i = (n) - 1; i >= 0; i--)
#define rreps(i, e, n) for(ll i = (n) - 1; i >= (e); i--)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((ll)(x).size())
#define len(x) ((ll)(x).length())

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    // ifstream in("input.txt");
    // cin.rdbuf(in.rdbuf());
    ll n, k;
    cin >> n >> k;
    k--;
    vector<ll> a(n);
    rep(i, n) cin >> a[i];
    if (a[k] == 0) {
        cout << 0 << endl;
        return 0;
    }
    ll right = 0, rmv = LONG_LONG_MAX;
    reps(i, k + 1, n) {
        rmv = min(rmv, a[i]);
        right += rmv;
        if (a[i] <= 1) break;
    }
    ll left = 0, lmv = LONG_LONG_MAX;
    rrep(i, k) {
        lmv = min(lmv, a[i]);
        left += a[i];
        if (a[i] <= 1) break;
    }
    ll ans;
    if (a[k] == 0) ans = 0;
    else if (a[k] == 1) ans = max(left, right) + 1;
    else ans = left + right + a[k];
    cout << ans << endl;
    return 0;
}