学步园

#include<iostream>
#include<cstdio>
using namespace std;
inline int read(){
    int x=0;char ch=getchar();
    while(ch<'0'||ch>'9')ch=getchar();
    while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
    return x;
}

int n,L,x,q[50005],l,r;
long long sum[50005],f[50005],num[50005];
inline double sqr(double x){return x*x;}
inline long long sqr2(long long x){return x*x;}
/*inline double calc(int k,int j){
     return (double)(f[j]-f[k]+sqr((double)sum[j])-sqr((double)sum[k]))/(double)(sum[j]-sum[k]);
}*/
long long A(int x)
{
     return sum[x]+x;
}
long double calc(int j,int k)  //k,i
{
     return (long double) (f[j]-f[k]+A(j)*A(j)-A(k)*A(k))/(long double) (A(j)-A(k));
}
int main(){
    n=read();L=read();
    for(int i=1;i<=n;i++)
    {scanf("%lld",&num[i]);sum[i]=sum[i-1]+num[i];}
    for(int i=1;i<=n;i++){
        while(l<r && calc(q[l],q[l+1])<2*(A(i)-1-L))l++;
        int now=q[l];
        f[i]=f[now]+(sum[i]-sum[now]+i-now-1-L)*(sum[i]-sum[now]+i-now-1-L);
       while(l<r && calc(q[r-1],q[r])>calc(q[r],i))r--;
        q[++r]=i;
    }
    printf("%lld",f[n]);
    return 0;    
}