分析：

很簡單的斜率優化，這裡不再贅述。

#include<cstdio>
#include<cstring>
#include<algorithm>
#define SF scanf
#define PF printf
#define MAXN 3010
using namespace std;
typedef long long ll;
ll dp[MAXN][MAXN];
ll q[MAXN],X;
ll s[MAXN];
int hd,tl,n,m;
int main(){
	SF("%d%d",&n,&m);
	for
 
(int i=1;i<=n;i++){
		SF("%lld",&s[i]);
		s[i]*=m;
		s[i]+=s[i-1];
	}
	X=s[n]/m;
	for(int i=1;i<=n;i++)
		dp[i][1]=(s[i]-X)*(s[i]-X);
	for(int i=2;i<=m;i++){
		hd=1,tl=0;
		for(int j=1;j<=n;j++){
			while(hd<tl&&dp[q[hd]][i-1]+s[q[hd]]*s[q[hd]]-dp[q[hd+1]][i-1]-s[q[hd+1]]*s[
 
q[hd+1]]>2ll*(s[j]-X)*(s[q[hd]]-s[q[hd+1]]))
				hd++;
//			PF("[%d %d %lld %lld (%d %d)]\n",j,i,q[hd],dp[j][i],hd,tl);
			if(hd>tl)
				dp[j][i]=-1;
			else
				dp[j][i]=(dp[q[hd]][i-1]+((s[j]-s[q[hd]])-X)*((s[j]-s[q[hd]])-X));
			if(dp[j][i-1]!=-1){
				while(hd<tl&&(dp[q[tl]][i-1]+
 
s[q[tl]]*s[q[tl]]-dp[q[tl-1]][i-1]-s[q[tl-1]]*s[q[tl-1]])*(s[j]-s[q[tl-1]])>(dp[j][i-1]+s[j]*s[j]-dp[q[tl-1]][i-1]-s[q[tl-1]]*s[q[tl-1]])*(s[q[tl]]-s[q[tl-1]]))
					tl--;
				q[++tl]=j;
			}
		}
	}
	PF("%lld\n",dp[n][m]/m);
}