#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
typedef long long ll;
typedef pair<ll,int> P;
const ll INF = 1e12;
const int MOD = 1000000007;


int main() {
        int n,k;
        cin >> n >> k;
        --k;
        vector<ll> a(n);
        rep(i,n) cin >> a[i];
        if(a[k] == 0) { cout << 0 << endl;}
        else if(a[k] == 1){
                ll res1 = 0,res2 = 0;
                for(int i = k; i >= 0;i--){
                        res1 += a[i];
                        if(a[i] == 1 && i != k) break;
                }
                for(int i = k;i < n;i++){
                        res2 += a[i];
                        if(a[i] == 1 && i != k) break;
                }
                cout << max(res1,res2) << endl;
        }
        else{
                ll res = 0;
                int l = 0,r = n-1;
                for(int i = k;i>=0;i--){
                        if(a[i] == 1) l = i;
                }
                for(int i = k;i<n;i++){
                        if(a[i] == 1) r = i;
                }
                for(int i=l;i<=r;i++){
                        res += a[i];
                }
                cout << res << endl;
        }


        return 0;
}