#include<stdio.h>
#define mare 10000010
long n,k,i,x,st,dr,q[5000000];
long long s;
long poz[5000000];
void rez()
{
	scanf("%ld %ld",&n,&k);
	st=1;
	q[dr]=mare;
	for(i=1;i<k;++i)
	{
		scanf("%ld",&x);
		while(x<=q[dr] && dr>=st) dr--;
		q[++dr]=x;
		poz[dr]=i;
	}
	
	for(i=k;i<=n;++i)
	{
		scanf("%ld",&x);
		
		while(x<=q[dr] && dr>=st) dr--;
		q[++dr]=x;
		poz[dr]=i;
		if(poz[st]<i-k+1)
		{
			st++;
		}
		s+=q[st];
		
	}
	printf("%lld\n",s);
}
int main()
{
	freopen("deque.in","r",stdin);
	freopen("deque.out","w",stdout);
	rez();
	return 0;
}