# coding: utf-8
# Your code here!
N,K=map(int,input().split())

def kill(lis):
    lis_index=len(lis)
    for i in range(len(lis)):
        #print(i)
        if lis[i]==0:
            lis_index==i
            break
        elif lis[i]==1:
            #print("a")
            lis_index=i
            break
    #print(lis[:i+1])
    return lis[:i+1]

A=[0]+list(map(int,input().split()))+[0]

L=A[:K]
#print(L[::-1][:100])
#print(len(kill(L[::-1])))


R=A[K+1:]
#print(len(R))
#print(len(kill(R)))

score=[sum(kill(L[::-1])),sum(kill(R))]

if A[K]>=2:
    print(sum(score)+A[K])
elif A[K]==1:
    print(max(score)+A[K])
else:
    print(0)