#include <bits/stdc++.h>
using namespace std;
 
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vvi = vector<vector<int>>;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
#define FOR(i, m, n) for (int i = (m); i < (n); i++)
#define FORR(i, m, n) for (int i = (m); i >= (n); i--)
#define REP(i, n) FOR(i, 0, (n))
#define REPR(i, n) FORR(i, (n) - 1, 0)
#define REP1(i, n) FOR(i, 1, (n) + 1)
#define REPS(c, s) for (char c : s)
#define ALL(c) (c).begin(), (c).end()
#define SORT(c) sort(ALL(c))
#define REV(c) reverse(ALL(c))
#define sz(v) (int)v.size()
#define endl '\n'
template<class T> inline bool chmin(T& a, T b) {if (a > b) {a = b; return true;} return false;}
template<class T> inline bool chmax(T& a, T b) {if (a < b) {a = b; return true;} return false;}
template<class T> inline void printv(vector<T>& v) {int n = sz(v); REP(i, n) cout << v[i] << (i == n - 1 ? endl : ' ');}
template<class T> inline void printlnv(vector<T>& v) {int n = sz(v); REP(i, n) cout << v[i] << endl;}
const int MOD = 1000000007;
const int INF = 1000000001;
const ll LINF = 1000000001000000001LL;
 
void solve();
 
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(numeric_limits<double>::max_digits10);
    solve();
    return 0;
}

void solve() {
    int n, k;
    cin >> n >> k;
    vl a(n + 2, 0);
    REP1(i, n) cin >> a[i];

    if (a[k] == 0) {
        cout << 0 << endl;
    } else if (a[k] == 1) {
        ll l = 0;
        for (int i = k - 1; i >= 0; i--) {
            if (a[i] == 0) break;
            if (a[i] == 1) {l += a[i]; break;}
            l += a[i];
        }
        ll r = 0;
        FOR(i, k + 1, n + 1) {
            if (a[i] == 0) break;
            if (a[i] == 1) {r += a[i]; break;}
            r += a[i];
        }
        cout << max(l, r) << endl;
    } else {
        ll l = 0;
        for (int i = k - 1; i >= 0; i--) {
            if (a[i] == 0) break;
            if (a[i] == 1) {l += a[i]; break;}
            l += a[i];
        }
        ll r = 0;
        FOR(i, k + 1, n + 1) {
            if (a[i] == 0) break;
            if (a[i] == 1) {r += a[i]; break;}
            r += a[i];
        }
        cout << a[k] + l + r << endl;

    }
}