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

int N, P;
long A[200000];

long sub(int l, int r) {
    long sum = 0;
    for (int i = l; i < r; ++i) {
        sum += A[i];
    }
    return sum;
}

int main() {
    cin >> N >> P; --P;
    for (int i = 0; i < N; ++i) cin >> A[i];
    
    int left = P - 1;
    while (left > 0 && A[left] >= 2) {
        --left;
    }
    
    int right = P + 1;
    while (right < N - 1 && A[right] >= 2) {
        ++right;
    }
    
    if (A[P] == 0) {
        cout << 0 << endl;
    } else if (A[P] == 1) {
        cout << max(sub(left, P + 1), sub(P, right + 1)) << endl;
    } else {
        cout << sub(left, right + 1) << endl;
    }
}