uses math,crt;
var i,n,x,m,t:longint;
a:array[0..1000000] of longint;
begin
clrscr;
readln(n,x);
for i:=1 to n do read(a[i]);
m:=0;
for i:=1 to x do m:=m+a[i];
t:=m;
for i:=2 to n-x+1 do
begin
t:=t-a[i-1]+a[i+x-1];
m:=max(m,t);
end;
writeln(m);
readkey;
end.