import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Scanner;

class Main {
	public static void main(String[] args) throws Exception {
		new Main().run();
	}
	
	void run() {
		Scanner sc=new Scanner(System.in);
		int N=sc.nextInt();
		int K=sc.nextInt();
		long[] A=new long[N];
		for(int i=0;i<N;++i) A[i]=sc.nextLong();
		--K;
		if(A[K]==0){
			System.out.println(0);
			return;
		}
		int r=K;
		int l=K;
		while(r+1<A.length&&A[r+1]>=1&&(r==K||A[r]==2))++r;
		while(l-1>=0&&A[l-1]>=1&&(l==K||A[l]==2))--l;
		long rc=0,lc=0;
		for(int i=K+1;i<=r;++i)rc+=A[i];
		for(int i=K-1;i>=l;--i)lc+=A[i];
		if(A[K]<=2){
			System.out.println(A[K]+Math.max(rc,lc));
		}else{
			System.out.println(A[K]+rc+lc);
		}
	}
	
	static void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}
	
}