# coding: utf-8
# Your code here!

import sys
readline = sys.stdin.readline
read = sys.stdin.read

n,k,*a = [int(i) for i in read().split()]
k -= 1

v = 0
rb = 0
for i in range(k+1,n):
    v += a[i]
    if a[i] <= 1:
        rb = v
        break
else:
    rb = v
        
v = 0
lb = 0
for i in range(k-1,-1,-1):
    v += a[i]
    if a[i] <= 1:
        lb = v
        break
else:
    lb = v

#print(r,rb)
#print(l,lb)

if a[k] == 0:
    print(0)
elif a[k] == 1:
    print(a[k]+max(lb,rb))
else:
    print(a[k]+lb+rb)