N,K=map(int,input().split())
K-=1
A=list(map(int,input().split()))
ans1=0
ans2=0
for i in range(K+1,N):
  if A[i]==0:
    break
  elif A[i]==1:
    ans1+=1
    break
  else:
    ans1+=A[i]
for i in range(K-1,-1,-1):
  if A[i]==0:
    break
  elif A[i]==1:
    ans2+=1
    break
  else:
    ans2+=A[i]
if A[K]==0:
  print(0)
elif A[K]==1:
  print(max(ans1,ans2)+A[K])
else:
  print(ans1+ans2+A[K])