#include<cstdio>
using namespace std;
int main()
{ freopen("deque.in","r",stdin);
  freopen("deque.out","w",stdout);
  int n,k,i,p,q,x,min=10000000,ok,mn=10000000;
  long long s;
  scanf("%d%d",&n,&k);
  for(i=1;i<=k;i++) { scanf("%d",&x);
					  if(x<min)min=x,p=i;
					  else if(x<mn)mn=x,q=i;
					}
  s=min;
  while(i<=n) { p+=k-1;ok=1;
				for(;i<=p&&ok;i++) { scanf("%d",&x);
									 if(x<min)min=x,p=i,ok=0;
									 else if(x<mn)mn=x,q=i;
									 s+=min;
									}
				if(ok)p=q,min=mn;
				mn=10000000;
			  }
  printf("%d",s);
  return 0;
}