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

if A[k-1] == 0 or A[k-1] == 1:
    print(A[k-1])
    exit()

A = [0] + A + [0]
#print(A)

for i in range(k):
    if A[i] == 0:
        p = i
        q = 0
    elif A[i] == 1:
        p = i
        q = 1

for i in reversed(range(k+1, n+2)):
    if A[i] == 0:
        r = i
        s = 0
    elif A[i] == 1:
        r = i
        s = 1

#print(p, q, r, s)
ans = sum(A[p:k])+A[k]+sum(A[k+1:r+1])
print(ans)