import sys
input=lambda: sys.stdin.readline().rstrip()
n,k=map(int,input().split())
A=[int(i) for i in input().split()]
if A[k-1]<=1:
  print(A[k-1])
else:
  ans=A[k-1]
  ans_l,ans_r=0,0
  l,r=1,1
  while k-1-l>=0:
    if A[k-1-l]>=2:
      ans_l+=A[k-1-l]
      l+=1
      continue
    else:
      ans_l+=A[k-1-l]
      break
  while k-1+r<n:
    if A[k-1+r]>=2:
      ans_r+=A[k-1+r]
      r+=1
      continue
    else:
      ans_r+=A[k-1+r]
      break
  print(ans+ans_l+ans_r)