from sys import stdin
def main():
    #入力
    readline=stdin.readline
    N,K=map(int,readline().split())
    A=list(map(int,readline().split()))
    
    if A[K-1]==0:
        print(0)

    elif A[K-1]>=1:
        left=0
        right=0
        now=K-1
        while now>0:
            now-=1
            left+=A[now]
            if A[now]==1:
                break
        now=K-1
        while now<N-1:
            now+=1
            right+=A[now]
            if A[now]==1:
                break
        
        if A[K-1]==1:
            print(A[K-1]+max(left,right))
        else:
            print(A[K-1]+left+right)


if __name__=="__main__":
    main()