import bisect import copy import decimal import fractions import heapq import itertools import math import random import sys import time from collections import Counter,deque,defaultdict from functools import lru_cache,reduce from heapq import heappush,heappop,heapify,heappushpop,_heappop_max,_heapify_max def _heappush_max(heap,item): heap.append(item) heapq._siftdown_max(heap, 0, len(heap)-1) def _heappushpop_max(heap, item): if heap and item < heap[0]: item, heap[0] = heap[0], item heapq._siftup_max(heap, 0) return item from math import gcd as GCD read=sys.stdin.read readline=sys.stdin.readline readlines=sys.stdin.readlines write=sys.stdout.write #import pypyjit #pypyjit.set_param('max_unroll_recursion=-1') #sys.set_int_max_str_digits(10**9) N,K=map(int,readline().split()) A=list(map(int,readline().split())) K-=1 l,r=max(0,K-1),min(N,K+2) while l and A[l]>=2: l-=1 while r=2: r+=1 if A[K]>=2: ans=sum(A[l:r]) elif A[K]==1: ans=max(sum(A[l:K+1]),sum(A[K:r])) else: ans=0 print(ans)