#include<stdio.h>
#define nmax 5000010
long n,k,p,u;
long v[nmax],deq[nmax];

void adauga(long i)
{
	while(u>=p && v[deq[u]]>=v[i])
		u--;
	deq[++u]=i;
}

void read()
{
	freopen("deque.in","r",stdin);
	freopen("deque.out","w",stdout);
	scanf("%ld%ld",&n,&k);
	long i,s=0;
	p=1;
	u=0;
	for(i=1;i<=k;i++)
	{
		scanf("%ld",&v[i]);
		adauga(i);
	}
	s=s+v[deq[p]];
	for(i=k+1;i<=n;i++)
	{
		scanf("%ld",&v[i]);
		if(deq[p]==i-k)
			++p;
		adauga(i);
		s=s+v[deq[p]];
	}
	printf("%ld\n",s);
}

int main()
{
	read();
	return 0;
}