import sys
input=sys.stdin.readline
def I(): return int(input())
def MI(): return map(int, input().split())
def LI(): return list(map(int, input().split()))

def main():
    mod=10**9+7
    N,K=MI()
    K-=1
    A=LI()
    if A[K]==0:
        print(0)
        exit()
    if min(A)>=2:
        print(sum(A))
    else:
        ans=A[K]
        for i in range(K+1,N):
            if A[i]==1:
                ans+=1
                break
            elif A[i]==0:
                break
            else:
                ans+=A[i]
        for i in range(K-1,-1,-1):
            if A[i]==1:
                ans+=1
                break
            elif A[i]==0:
                break
            else:
                ans+=A[i]
        print(ans)
            
        

        


main()