import sys
input = sys.stdin.readline
    
N,K=map(int,input().split())
A=list(map(int,input().split()))

K-=1

if A[K]==0:
    print(0)
elif A[K]==1:
    A1=0
    for i in range(K+1,N):
        A1+=A[i]
        if A[i]<=1:
            break
    A2=0
    for i in range(K-1,-1,-1):
        A2+=A[i]
        if A[i]<=1:
            break
    print(max(A1,A2)+1)
else:
    ANS=A[K]
    for i in range(K+1,N):
        ANS+=A[i]
        if A[i]<=1:
            break
    for i in range(K-1,-1,-1):
        ANS+=A[i]
        if A[i]<=1:
            break
    print(ANS)