#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define pb push_back
#define fi first
#define se second
typedef pair<ll,ll> P;
using VP = vector<P>; using VVP = vector<VP>;
using VI = vector<ll>; using VVI = vector<VI>; using VVVI = vector<VVI>;
const int inf=1e9+7;
const ll INF=1LL<<61;
const ll mod=1e9+7;

template<class T>
inline bool chmax(T &a, T b) {
    if(a < b) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
inline bool chmin(T &a, T b) {
    if(a > b) {
        a = b;
        return true;
    }
    return false;
}

int main(){
    int i,j;
    ll n,k;
    cin>>n>>k;
    ll a[n+1];
    ll ans=0;
    for(i=1;i<=n;i++) cin>>a[i];
    if(a[k]==0){
        cout<<0<<endl;
        return 0;
    }
    if(a[k]==1){
        ll ans=0;
        ll cnt=0;
        for(i=k+1;i<=n;i++){
            if(a[i]>=2) cnt+=a[i];
            else  {
                cnt+=a[i];
                break;
            }
        }
        ans=max(ans,1+cnt);
        cnt=0;
        for(i=k-1;i>=1;i--){
            if(a[i]>=2) cnt+=a[i];
            else  {
                cnt+=a[i];
                break;
            }
        }
        ans=max(ans,1+cnt);
        cout<<ans<<endl;
        return 0;
    }
    else{
        ll ans=0;
        ll cnt=0;
        for(i=k+1;i<=n;i++){
            if(a[i]>=2) cnt+=a[i];
            else  {
                cnt+=a[i];
                break;
            }
        }
        for(i=k-1;i>=1;i--){
            if(a[i]>=2) cnt+=a[i];
            else  {
                cnt+=a[i];
                break;
            }
        }
        ans=max(ans,a[k]+cnt);
        cout<<ans<<endl;
        return 0;
    }
}