import java.util.*;

class Main{
    public static void main(String[] args){
        Scanner sc=new Scanner(System.in);
        int N=sc.nextInt();
        int K=sc.nextInt();
        long[] A=new long[N];
        
        long ans=0;
        
        for(int i=0;i<N;i++){
            A[i]=sc.nextLong();
        }
        
        if(A[K-1]==0L){
            System.out.println(0);
        }else if(A[K-1]==1L){
            for(int i=0;i<K-1;i++){
                if(A[i]<=1){
                    ans=A[i];
                }else{
                    ans+=A[i];
                }
            }
            long ANS=ans;
            ans=0;
            for(int i=K;i<N;i++){
                if(A[i]<=1){
                    ans+=A[i];
                    break;
                }else{
                    ans+=A[i];
                }
            }
            if(ANS>ans){
                System.out.println(ANS);
            }else{
                System.out.println(ans);
            }
        }else{
            for(int i=0;i<N;i++){
                boolean through=false;
                if(A[i]<=1&&!through){
                    ans=A[i];
                }else if(A[i]<=1&&through){
                    ans+=A[i];
                    System.out.println(ans);
                    return;
                }else{
                    ans+=A[i];
                }
            }
            System.out.println(ans);
        }
    }
}