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

#define int long long

int n, k;
int a[100010];

signed main(){
	cin >> n >> k;

	for(int i = 0;i < n;i++){
		cin >> a[i];
	}
	k--;

	vector<int> lv, rv;
	int ans = 0;

	for(int i = 0;i < k;i++){
		if(a[i] <= 1) lv.clear();
		if(a[i] > 0) lv.push_back(a[i]);
	}

	int left = 0;
	int right = 0;
	int noko = 0;
	if(lv.size() > 0){
		for(int i = 0;i < lv.size();i++){
			left += lv[i];
		}
	}

	for(int i = n-1;i > k;i--){
		if(a[i] <= 1) rv.clear();
		if(a[i] > 0) rv.push_back(a[i]);
	}

	if(rv.size() > 0){
		for(int i = 0;i < rv.size();i++){
			right += rv[i];
		}
	}

	if(a[k] == 0){
		cout << 0 << endl;
	}else if(a[k] == 1){
		cout << max(left, right)+1 << endl;
	}else{
		cout << left + right + a[k] << endl;
	}

	return 0;
}