#include<bits/stdc++.h>
using namespace std;
const int N=1005;
int a[N];
int n,m;
int main(){
	scanf("%d%d",&n,&m);
	for (int i = 1;i <= n;++i)scanf("%d",&a[i]);
	int ans=10000000;
	for (int i = 1;i <= n-m+1;++i){
		int flag=a[i]+=a[i+1]+=a[i+2];
		if (flag < ans){
			ans=flag;
		}
	}
	printf("%d",ans);
	return 0;
}