import sys data = sys.stdin.buffer.read().split() n = int(data[0]) k = int(data[1]) waters = list(map(int, data[2:2 + n])) window_sum=0 len_waters = len(waters) max_waters = 0 left=0 right=0 while right<len_waters: c=waters[right] window_sum+=c right+=1 while right-left>k+1: window_sum-=waters[left] left+=1 max_waters=max(max_waters,window_sum) print(max_waters)