#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> l_l;
typedef pair<int, int> i_i;
template<class T>
inline bool chmax(T &a, T b) {
    if(a < b) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
inline bool chmin(T &a, T b) {
    if(a > b) {
        a = b;
        return true;
    }
    return false;
}

const long double EPS = 1e-10;
const long long INF = 1e18;
const long double PI = acos(-1.0L);
//const ll mod = 1000000007;
int N, K;
vector<ll> A;
ll ans;

int main() {
    //cout.precision(10);
    cin.tie(0);
    ios::sync_with_stdio(false);
    cin >> N >> K;
    K--;
    A.resize(N);
    for(int i = 0; i < N; i++) cin >> A[i];
    ans = A[K];
    ll r = 0;
    for(int i = K + 1; i < N; i++) {
        r += A[i];
        if(A[i] <= 1) break;
    }
    ll l = 0;
    for(int i = K - 1; i >= 0; i--) {
        l += A[i];
        if(A[i] <= 1) break;
    }
    if(A[K] == 0) ans = 0;
    if(A[K] == 1) ans = A[K] + max(l, r);
    if(A[K] >= 2) ans = A[K] + l + r;
    cout << ans << endl;
    return 0;
}