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