# a,b,c=map(int,input().split())
# n=int(input())
# t = (c+n)%60
# l=(c+n)//60
# m=(b+l)%60
# h=a+(b+l)//60
# print(h%24,m,t)
'''
a=int(input())
b=list(map(int,input().split()))
t=0
c=0
for i in range(a):
if b[i]==1:
c=c+1
t=t+c
else:
t=t+0
c=0
print(t)
'''
'''
a[0] ~ a[4]의 합
sum(a[:5])
a[5] ~ 마지막까지
sum(a[5:])
a[2] ~ a[4]
sum(a[2:5])
'''
'''
a,b=map(int,input().split())
l=list(map(int,input().split()))
list=[]
for i in range(a-b+1):
list.append(sum(l[i:i+b]))
print(max(list))
'''
a,b=map(int,input().split())
l=list(map(int,input().split()))
list=[]
t=0
for i in range(a-b+1):
t=sum(l[0:b+1])
list.append(t-l[i-2]+l[i])
print(max(list))