#include <stdio.h>
#include <string>
#include <cstring>
#include <stdlib.h>
#include <math.h>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <list>
#include <iterator>
#include <assert.h>
#pragma warning(disable:4996) 

typedef long long ll;
#define MIN(a, b) ((a)>(b)? (b): (a))
#define MAX(a, b) ((a)<(b)? (b): (a))
#define LINF 9223300000000000000
#define INF 2140000000
const long long MOD = 1000000007;
//const long long MOD = 998244353;

using namespace std;


void solve()
{
    int n,K;
    scanf("%d%d", &n, &K); K--;
    vector<int> a(n);
    int i;
    for(i=0; i<n; i++) {
        scanf("%d", &a[i]);
    }
    ll sum=a[K];
    ll sum0=0, sum1=0;
    for(i=K-1; i>=0; i--) {
        if(a[i]<=1) {
            sum0+=a[i];
            break;
        }
        else {
            sum0+=a[i];
        }
    }
    for (i = K + 1; i < n; i++) {
        if(a[i]<=1) {
            sum1+=a[i];
            break;
        }
        else {
            sum1+=a[i];
        }
    }
    if(a[K]==0) {
        printf("%lld\n", sum);
    }
    else if(a[K]==1) {
        printf("%lld\n", sum+MAX(sum0,sum1));
    }
    else {
        printf("%lld\n", sum+sum0+sum1);
    }

    return;
}


int main(int argc, char* argv[])
{
#if 1
	solve();
#else
	int T; scanf("%d", &T);
	while (T--) {
		solve();
	}
#endif
	return 0;
}