# 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
r = rb = -1
for i in range(k+1,n):
    v += a[i]
    if a[i] == 0:
        r = v
        if rb == -1: rb = v
        break
    elif a[i] == 1:
        rb = v
        
v = 0
l = lb = -1
for i in range(k-1,-1,-1):
    v += a[i]
    if a[i] == 0:
        l = v
        if lb == -1: lb = v
        break
    elif a[i] == 1:
        lb = v

if r == -1: r = 0
if l == -1: l = 0
if rb == -1: rb = 0
if lb == -1: lb = 0

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

if a[k] == 0:
    print(0)
else:
    print(a[k]+lb+rb)