#include <bits/stdc++.h>
using namespace std;
 
int main()
{
	int N, K;
	cin >> N >> K;
	vector<long long> A( K );
	for( int i = 0; i < N; i++ ) {
		cin >> A[i];
	}

	K--;
	long long ans1 = 0;
	for( int i = K + 1; i < N; i++ ) {
		if( A[i] < 1 ) break;
		ans1 += A[i];
		if( A[i] < 2 ) break;
	}
	long long ans2 = 0;
	for( int i = K - 1; i >= 0; i-- ) {
		if( A[i] < 1 ) break;
		ans2 += A[i];
		if( A[i] < 2 ) break;
	}
	long long ans = 0;
	if( A[K] == 1 ) max( ans1 + A[K], ans2 + A[K] );
	else if( A[K] > 1 ) ans = ans1 + ans2 + A[K];

	cout << ans << endl;
}