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

int main() {
    int n, k;
    cin >> n >> k;
    vector<int> a(n);
    rep(i, n) cin >> a[i];

    long result = a[k-1];
    long left = 0, right = 0;
    for(int i=k-2; i>=0; i--) {
        left += a[i];
        if(a[i] <= 1) {
            break;
        }
    }
    for(int i=k; i<n; i++) {
        right += a[i];
        if(a[i] <= 1) {
            break;
        }
    }

    if(result == 0) {
        cout << "0\n";
    } else if(result == 1) {
        result += left > right ? left : right;
        cout << result << "\n";
    } else {
        cout << result + left + right << "\n";
    }
}