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<N and A[r-1]>=2:
    r+=1
if A[K]>=2:
    ans=sum(A[l:r])
elif A[K]==1:
    ans=min(A[l:K+1],A[K:r])
else:
    ans=0
print(ans)