#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
int n,k,f[100010],ans=99999999;
int desl(int b){
	int u=0;
	for(int i=b;i<b+k-1;i++){
		u+=(f[i+1]-f[i])*((i)*(b+k-1-i));	
	}
	return u;
}
int main(){
	 freopen("qwa.in","r",stdin);
     freopen("qwa.out","w",stdout);
	scanf("%d%d",&n,&k);
	for(int i=1;i<=n;i++)scanf("%d",&f[i]);
	for(int i=1;i<=n-k+1;i++){
		ans=min(ans,desl(i));
	}
	printf("%d",ans);
	return 0;
}