#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <cmath>

#define ll long long 

using namespace std;

ll n, k, sum = 0, maxn = 0, ans = 0;
ll a[1000010];

int main(){
	scanf("%lld%lld", &n, &k);
	
	for(ll i = 1; i <= n - 1; i++){
		scanf("%lld", &a[i]);
		ans += a[i];
	}
	for(ll i = 1; i <= k; i++){
		sum += a[i];
		maxn = sum;
	}
	for(ll i = k + 1; i <= n - k; i++){
		sum = sum + a[i] - a[i - k];
		if(sum > maxn) maxn = sum;
	}
	printf("%lld", ans - maxn);
}