import sys

sys.setrecursionlimit(10 ** 6)
from bisect import *
from collections import *
from heapq import *

def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def SI(): return sys.stdin.readline()[:-1]
def LLI(rows_number): return [LI() for _ in range(rows_number)]
int1 = lambda x: int(x) - 1
def MI1(): return map(int1, sys.stdin.readline().split())
def LI1(): return list(map(int1, sys.stdin.readline().split()))
p2D = lambda x: print(*x, sep="\n")
dij = [(1, 0), (0, 1), (-1, 0), (0, -1)]

def main():
    n,k=MI()
    aa=LI()
    k-=1
    if aa[k]==0:
        print(0)
        exit()
    ls=rs=0
    for i in range(k-1,-1,-1):
        ls+=aa[i]
        if aa[i]<2:break
    for i in range(k+1,n):
        rs+=aa[i]
        if aa[i]<2:break
    if aa[k]==1:print(max(ls,rs)+1)
    else:print(ls+rs+aa[k])

main()