#include<bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(),x.end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define INF 1000000000
#define mod 1000000007
using ll=long long;
const ll LINF=1001002003004005006ll;
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}

signed main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    int n,k;cin>>n>>k;k--;
    vector<ll> a(n);
    rep(i,n){
        cin>>a[i];
    }

    if(a[k]==0){
        cout<<0<<endl;
        return 0;
    }

    int l=k-1,r=k+1;//()
    while(r<n and a[r]>1) r++;
    while(l>=0 and a[l]>1) l--;

    ll suml=0;
    if(l>=0) suml+=a[l];
    for(int i=l+1;i<k;i++) suml+=a[i];

    ll sumr=0;
    if(r<n) sumr+=a[r];
    for(int i=r-1;i>k;i--) sumr+=a[i];

    if(a[k]==1) cout<<max(suml,sumr)+a[k]<<endl;
    else if(a[k]>1) cout<<suml+sumr+a[k]<<endl;
    else cout<<0<<endl;
    return 0;
}