n,k,*a=map(int,open(0).read().split())
l=r=0
m=a[k-1]
for i in a[k:]:
    r+=i
    if i<2:
        break
for i in a[:k-1][::-1]:
    l+=i
    if i<2:
        break
if m==0:
    print(0)
elif m==1:
    print(max(l,r)+m)
else:
    print(l+r+m)