#include<fstream.h>
ifstream fin("deque.in");
ofstream fout("deque.out");
long a[5000010],c[5000010],n,k,i,ic,sc;
double s;
int main()
	{fin>>n>>k;
	for (i=1;i<=n;i++)
		fin>>a[i];
	sc=0;ic=1;
	for (i=1;i<=n;i++)
		{while (ic<=sc&&a[c[sc]]>=a[i]) sc--;
		 sc++;
		 c[sc]=i;
		 if (c[ic]==i-k) ic++;
		 if (i>=k)
			s=s+a[c[ic]];
		 }
	fout<<s;
fout.close();
return 0;}