//#include <bits/stdc++.h>
#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
//#include "boost/multiprecision/cpp_int.hpp"
//typedef boost::multiprecision::cpp_int ll;
typedef long double dd;
#define i_7 (ll)(1E9+7)
//#define i_7 998244353
#define i_5 i_7-2
ll mod(ll a){
    ll c=a%i_7;
    if(c>=0)return c;
    return c+i_7;
}
typedef pair<ll,ll> l_l;
typedef pair<dd,dd> d_d;
ll inf=(ll)1E16;
#define rep(i,l,r) for(ll i=l;i<=r;i++)
#define pb push_back
ll max(ll a,ll b){if(a<b)return b;else return a;}
ll min(ll a,ll b){if(a>b)return b;else return a;}
void Max(ll &pos,ll val){pos=max(pos,val);}//Max(dp[n],dp[n-1]);
void Min(ll &pos,ll val){pos=min(pos,val);}
void Add(ll &pos,ll val){pos=mod(pos+val);}
dd EPS=1E-9;
#define fastio ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
#define fi first
#define se second
///////////////////////////

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