#define  _CRT_SECURE_NO_WARNINGS
#pragma comment (linker, "/STACK:526000000")

#include "bits/stdc++.h"

using namespace std;
typedef string::const_iterator State;
#define eps 1e-11L
#define MAX_MOD 1000000007LL
#define GYAKU 500000004LL

#define MOD 998244353LL
#define seg_size 262144 * 4LL
#define pb push_back
#define mp make_pair
typedef long long ll;
#define REP(a,b) for(long long (a) = 0;(a) < (b);++(a))
#define ALL(x) (x).begin(),(x).end()

void init() {
    iostream::sync_with_stdio(false);
    cout << fixed << setprecision(20);
}

#define int ll
void solve(){
    int n, k;
    cin >> n >> k;
    k--;
    vector<int> inputs(n);
    REP(i, n) {
        cin >> inputs[i];
    }
    if (inputs[k] == 0) {
        cout << 0 << endl;
        return;
    }
    int base_ans = 0;
    int now = 0;
    int ans[2] = {};
    for (int i = k; i >= 0; --i) {
        if (inputs[i] == 0)break;
         ans[0] += inputs[i];
        
        if (i!=k&&inputs[i] == 1) {
            break;
        }
    }
    now = 0;
    for (int i = k; i < inputs.size(); ++i) {
        if (inputs[i] == 0)break;
        ans[1] += inputs[i];
        if (i != k&&inputs[i] == 1) {
            break;
        }
    }
    if (inputs[k] == 1) {
        cout << max(ans[0], ans[1]) << endl;
    }
    else {
        cout << ans[0] + ans[1] - inputs[k] << endl;
    }
}

#undef int
int main() {
    init();

    solve();
}