N,K = map(int,input().split())
lsA = list(map(int,input().split()))
K -= 1
if lsA[K] == 0:
    print(0)
elif lsA[K] == 1:
    a1 = 0
    ind = K+1
    while ind < N:
        if lsA[ind] == 0:
            break
        elif lsA[ind] == 1:
            a1 += 1
            break
        else:
            a1 += lsA[ind]
            ind += 1
    a2 = 0
    ind = K-1
    while 0 <= ind:
        if lsA[ind] == 0:
            break
        elif lsA[ind] == 1:
            a2 += 1
            break
        else:
            a2 += lsA[ind]
            ind -= 1
    print(max(a1,a2))
else:
    a1 = 0
    ind = K+1
    while ind < N:
        if lsA[ind] == 0:
            break
        elif lsA[ind] == 1:
            a1 += 1
            break
        else:
            a1 += lsA[ind]
            ind += 1
    a2 = 0
    ind = K-1
    while 0 <= ind:
        if lsA[ind] == 0:
            break
        elif lsA[ind] == 1:
            a2 += 1
            break
        else:
            a2 += lsA[ind]
            ind -= 1
    print(a1+a2+lsA[K])