#include<bits/stdc++.h>
using namespace std;
#define fs first
#define sc second
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define ALL(A) A.begin(),A.end()
#define RALL(A) A.rbegin(),A.rend()
typedef long long LL;
typedef pair<LL,LL> P;
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
template<typename T> T gcd(T a,T b){return b?gcd(b,a%b):a;}
const LL mod=1000000007;
const LL LINF=1LL<<60;
const int INF=1<<30;
int dx[]={1,0,-1,0,1,-1,1,-1};
int dy[]={0,1,0,-1,1,-1,-1,1};


int main(){
    int n,k;cin >> n >> k;
    k--;
    vector<LL> a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    LL r = 0, l = 0;
    for (int i = k+1; i < n; i++) {
        r += a[i];
        if(a[i] <= 1){
            break;
        }
    }
    for (int i = k-1; i >= 0; i--) {
        l += a[i];
        if(a[i] <= 1){
            break;
        }
    }
    if(a[k] == 0) cout << 0 << endl;
    else if(a[k] == 1) cout << max(l, r) + a[k] << endl;
    else cout << l + r + a[k] << endl;
    return 0;
}