[n,m] = list(map(int,input().split()))
lst = []
for times in range(n):
    lst.append(int(input()))
minimun = 0
for i in range(0,n-m+1):
    summ = sum(lst[i:i+m])
    if minimun == 0:
        minimun = summ
    minimun = min(minimun,summ)
print(minimun)