【CodeForces 613A】Peter and Snow Blower

#include <stdio.h>
#include<iostream>
#include <cmath>
#define dd double
using namespace std;

int n;
dd far,nea=1e19,t,P=acos(-1.0);

struct po
{
    double x,y;
} p[100005];

dd P_P(po a,po b)
{
    return (a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y);
}

dd P_L(po a,po b,po c)//point c --> line ab
{
    dd l1=P_P(a,b);
    dd l2=P_P(a,c);
    dd l3=P_P(b,c);

    if(l2*l2 >= (l1*l1+l3*l3))
        return l3;

    if(l3*l3>=(l2*l2+l1*l1))
        return l2;

    dd A=a.y-b.y;
    dd B=b.x-a.x;
    dd C=a.x*b.y - a.y*b.x;
//line ab: Ax+By+C=0;
    dd D=A*c.x + B*c.y + C;

    return fabs(D*D/(A*A+B*B));
}

int main()
{
    scanf("%d",&n);
    for(int i=0;i<=n;i++)
        scanf("%lf%lf",&p[i].x,&p[i].y);

    for(int i=1;i<=n;i++)
    {
        if (i<n) nea = min( P_L(p[i], p[i+1], p[0]), nea);
        else nea = min( P_L(p[n], p[1], p[0]), nea);

        far=max(far,P_P(p[0],p[i]));
    }
    printf("%.18lf\n",P*(far-nea));

    return 0;
}