学步园

#include<algorithm>
#include<iostream>
#include<cstdio>
#include<cmath>
#define inf 0x7fffffff
using namespace std;
inline int read(){
    int x=0,f=1;char ch=getchar();
    while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
    while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
    return x*f;
}
struct point{
	int x,y;
}p[50001],s[50001];
int n,top;
inline int sqr(int x){return x*x;}
int dis(point a,point b){return (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y);}  
int mul(point p1,point p2,point p0){return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);}  
inline bool cmp(point a,point b){if(!mul(p[0],a,b))return dis(a,p[0])<dis(b,p[0]);else return mul(p[0],a,b)>0;}  
void graham(){
	top=2;int k=0;point t;
	for(int i=1;i<n;i++)
	    if((p[k].y>p[i].y)||(p[k].y==p[i].y&&p[k].x>p[i].x))k=i;
	t=p[0];p[0]=p[k];p[k]=t;
	sort(p+1,p+n,cmp);
	s[0]=p[0],s[1]=p[1],s[2]=p[2];
	for(int i=3;i<n;i++){
		while(top&&mul(p[i],s[top],s[top-1])>=0)top--;
		s[++top]=p[i];
	}
	s[++top]=p[0];
}
int convex_diameter(){
	double maxd=0.0;
	if(top==1)return maxd;
	#define next(i) ((i+1)%top)
	for(int i=0,j=1;i<top;i++){
		while(abs(mul(s[next(i)],s[j],s[i]))-abs(mul(s[next(i)],s[next(j)],s[i]))<0)j=next(j);
		if(dis(s[i],s[j])>maxd)maxd=dis(s[i],s[j]);
		if(dis(s[next(i)],s[next(j)])>maxd)maxd=dis(s[next(i)],s[next(j)]);
	}
	return maxd;
}
int main(){   
    scanf("%d",&n);  
    for(int i=0;i<n;i++)scanf("%d%d",&p[i].x,&p[i].y);  
    graham();  
    cout<<convex_diameter()<<endl;
    return 0;  
}