#include<bits/stdc++.h>
using namespace std;
typedef long long ll;


signed main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << fixed << setprecision(20);

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

}