#include <bits/stdc++.h>
using namespace std;
template <class T> inline bool chmax(T &a, T b) {
    if(a < b) {
        a = b;
        return 1;
    }
    return 0;
}
template <class T> inline bool chmin(T &a, T b) {
    if(a > b) {
        a = b;
        return 1;
    }
    return 0;
}
typedef long long int ll;

#define ALL(v) (v).begin(), (v).end()
#define RALL(v) (v).rbegin(), (v).rend()
#define endl "\n"
const double EPS = 1e-7;
const int INF = 1 << 30;
const ll LLINF = 1LL << 60;
const double PI = acos(-1);
const int MOD = 1000000007;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};

//-------------------------------------

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n, k;
    cin >> n >> k;
    k--;
    vector<ll> a(n);
    for(int i = 0; i < n; i++) {
        cin >> a[i];
    }
    if(a[k] == 0) {
        cout << 0 << endl;
    } else {
        int l, r;
        for(r = k; r < n - 1; r++) {
            if(a[r + 1] == 1) {
                r++;
                break;
            } else if(a[r + 1] == 0) {
                break;
            }
        }
        for(l = k; l > 0; l--) {
            if(a[l - 1] == 1) {
                l--;
                break;
            } else if(a[l - 1] == 0) {
                break;
            }
        }
        ll suml = 0, sumr = 0;
        for(int i = l; i < k; i++) {
            suml += a[i];
        }
        for(int i = k + 1; i <= r; i++) {
            sumr += a[i];
        }
        // cout << l << " " << r << endl;
        if(a[k] == 1) {
            cout << max(suml, sumr) + 1 << endl;
        } else {
            cout << suml + sumr + a[k] << endl;
        }
    }
}