N,K=map(int,input().split())
L=[0]+list(map(int,input().split()))+[0]

if L[K]==0:
    print(0)
    exit()
else:
    right=L[K]
    for r in range(K+1,10**9):
        if L[r]>1:
            right+=L[r]
        if L[r]==1:
            right+=1
            break
        if L[r]==0:
            break
    left=L[K]
    for l in range(1,10**9):
        if L[K-l]>1:
            left+=L[K-l]
        if L[K-l]==1:
            left+=1
            break
        if L[K-l]==0:
            break
    if L[K]==1:
        print(max(left,right))
        #print(left,right)
        #print(L)
    else:
        print(left+right-L[K])
        #print(left,right)