N,K=map(int, input().split())
A=list(map(int, input().split()))
lcnt,rcnt=0,0
K-=1 
if A[K]==0:
    print(0)
    exit()

for i in range(K):
    if A[K-1-i]==0:
        break
    elif A[K-1-i]==1:
        lcnt+=1 
        break
    else:
        lcnt+=A[K-1-i]
for i in range(N-K-1):
    if A[K+1+i]==0:
        break
    elif A[K+1+i]==1:
        rcnt+=1 
        break
    else:
        rcnt+=A[K+1+i] 
if A[K]==1:
    print(max(rcnt,lcnt)+1)
else:
    print(rcnt+lcnt+A[K])