import sys data = sys.stdin.buffer.read().split() n = int(data[0]) k = int(data[1]) waters = list(map(int, data[2:2 + n])) len_waters = len(waters) pre_sum=[0]*(len_waters+1) for i in range(1,len_waters+1): pre_sum[i]=pre_sum[i-1]+waters[i-1] max_water=0 for p in range(1,len_waters+1): start=max(1,p-k) total = pre_sum[p] - pre_sum[start - 1] max_water = max(max_water, total) print(max_water)