#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <tuple>
#include <deque>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <complex>
using namespace std;

#define chmax(x, y) x = max(x, y)
#define chmin(x, y) x = min(x, y)

typedef long long ll;
typedef pair<int, int> P;
typedef pair<int, double> Pid;
typedef pair<double, int> Pdi;
typedef pair<ll, int> Pl;
typedef pair<int, pair<int, int>> PP;
constexpr double PI = 3.1415926535897932;   // acos(-1)
constexpr double EPS = 1e-10;
constexpr int INF = 1001001001;
constexpr int mod = 1e+9 + 7;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    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;
        return 0;
    }
    int pos = k;
    ll left = 0;
    while(pos > 0){
        --pos;
        if(pos == 0){
            left += a[0];
            break;
        }
        if(a[pos] <= 1){
            left += a[pos];
            break;
        }
        left += a[pos];
    }
    pos = k;
    ll right = 0;
    while(pos < n - 1){
        ++pos;
        if(pos == n - 1){
            right += a[n - 1];
            break;
        }
        if(a[pos] <= 1){
            right += a[pos];
            break;
        }
        right += a[pos];
    }
    if(a[k] == 1){
        cout << max(left, right) + a[k] << endl;
    }
    else{
        cout << left + right + a[k] << endl;
    }
}