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

if A[K]==0:
    res=0
elif A[K]==1:
    res=max(Sl,Sr)+1
else:
    res=Sl+Sr+A[K]
print(res)