#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("YES");}else{puts("NO");}
#define MAX_N 200005

int main() {
    ll n, k;
    cin >> n >> k;
    k--;
    ll a[n];
    rep(i,n)cin >> a[i];

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

    ll ans = a[k];
    ll tmp1 = 0;
    ll tmp2 = 0;

    int i = k - 1;

    while(i >= 0){
        if(a[i] >= 2){
            tmp1 += a[i];
            i--;
        }else if(a[i] == 1){
            tmp1++;
            break;
        }else{
            break;
        }
    }

    i = k + 1;
    while(i < n){
        if(a[i] >= 2){
            tmp2 += a[i];
            i++;
        }else if(a[i] == 1){
            tmp2++;
            break;
        }else{
            break;
        }
    }

    if(a[k] == 1){
        ans += max(tmp1, tmp2);
    }else{
        ans += tmp1 + tmp2;
    }

    cout << ans << endl;
    return 0;
}