#include "bits/stdc++.h"
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;

using ll = long long;
const ll MOD = 1e9 + 7;

int main() {
    int N, K;
    cin >> N >> K;
    K--;
    vector<int> A(N);
    rep(i, N) cin >> A[i];
    if (A[K] == 0) {
        cout << "0\n";
        return 0;
    }
    ll l = 0, r = 0;
    for (int i = K - 1; i >= 0; i--) {
        if (A[i] < 2) {
            l += A[i];
            break;
        }
        l += A[i];
    }
    for (int i = K + 1; i < N; i++) {
        if (A[i] < 2) {
            r += A[i];
            break;
        }
        r += A[i];
    }
    if (A[K] == 1) {
        if (l > r) cout << l + A[K] << endl;
        else cout << r + A[K] << endl;
        return 0;
    }
    cout << l + A[K] + r << endl;
}