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