n, k = list(map(int, input().split()))
a = list(map(int, input().split()))

l_sum = 0
r_sum = 0
for i in range(k, n):
    if a[i] == 0:
        break
    elif a[i] == 1:
        r_sum += 1
        break
    else:
        r_sum += a[i]

for i in range(k - 2, -1, -1):
    if a[i] == 0:
        break
    elif a[i] == 1:
        l_sum += 1
        break
    else:
        l_sum += a[i]
        
ans = a[k - 1]
if a[k - 1] == 1:
    ans += max(l_sum, r_sum)
if a[k - 1] > 1:
    ans += l_sum + r_sum
print(ans)