#include <iostream>
#include <utility>
#include <vector>
#include <algorithm>
#define llint long long

using namespace std;
typedef pair<llint, llint> P;

llint n, k;
llint a[200005];

int main(void)
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	cin >> n >> k;
	for(int i = 1; i <= n; i++) cin >> a[i];
	
	if(a[k] == 0){
		cout<< 0 << endl;
		return 0;
	}
	
	llint lsum = 0;
	for(int i = k-1; i >= 1; i--){
		if(a[i] == 0) break;
		lsum += a[i];
		if(a[i] <= 1) break;
	}
	llint rsum = 0;
	for(int i = k+1; i <= n; i++){
		if(a[i] == 0) break;
		rsum += a[i];
		if(a[i] <= 1) break;
	}
	
	if(a[k] == 1){
		cout << max(lsum, rsum) + 1 << endl;
		return 0;
	}
	
	cout << lsum + rsum + a[k] << endl;
	
	return 0;
}