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

using namespace std;

typedef long long ll;

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	
	int n, k;
	cin >> n >> k;
	k--;

	vector<ll> a(n);
	rep(i, n) cin >> a[i];

	if(a[k] == 0){
		cout << 0 << endl;
		return 0;
	}

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

	for(int i = k + 1; i < n; i++){
		right += a[i];
		if(a[i] <= 1) break;
	}

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