#include <bits/stdc++.h>
using namespace std;

int main(){
    int n, k;
    cin >> n >> k;
    vector<int64_t> A(n);
    copy_n(istream_iterator<int64_t>(cin), n, A.begin());
    k--;

    if (A[k] == 0){
        puts("0");
        return 0;
    }

    int64_t f = 0;
    for (int i = k - 1; i >= 0; i--){
        f += A[i];
        if (A[i] < 2)
            break;
    }

    int64_t b = 0;
    for (int i = k + 1; i < n; i++){
        b += A[i];
        if (A[i] < 2)
            break;
    }

    if (A[k] == 1){
        cout << max(f, b) + A[k] << endl;
    } else {
        cout << f + b + A[k] << endl;
    }
}