WA代码(把最后输出改成%.0f就AC了)：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
using namespace std;
const int Max = 1100;

struct Point
{
    int x;
    int y;
};
int num;
Point p[Max];   ///原始点集
Point ch[Max];  ///凸包点集

///p0p1叉乘p0p2
int xmult(const Point& p0, const Point& p1, const Point& p2)
{
    return (p1.x-p0.x)*(p2.y-p0.y) - (p2.x-p0.x)*(p1.y-p0.y);
}

double dis(const Point& a, const Point& b)
{
    return sqrt((double)(a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}

///逆时针顺序把极角排序
bool cmp(const Point& a, const Point& b)
{
    int ret = xmult(p[0],a,b);
    if(ret>0)
        return true;
    if(ret==0&&dis(p[0],a)<dis(p[0],b))
        return true;
    return false;
}

/**
p: 原始点序列
n: 原始点的个数
ch: 栈,用来保存凸包上的点
top: 凸包上点的个数
*/
void Graham(Point* p, int n, Point* ch, int& top)
{
    int i,k=0;
    for(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;
    }
    swap(p[k],p[0]);
    sort(p+1,p+n,cmp);
    top = 0;
    ch[top++] = p[0];
    ch[top++] = p[1];
    ch[top++] = p[2];
    for(i = 3; i < n; ch[top++]=p[i++])
        while(top>=2 && xmult(ch[top-2],ch[top-1],p[i])<0)    ///是否向左
            top--;
}

int main()
{
    int i;
    int n,l;
    while(scanf("%d %d",&n,&l)!=EOF)
    {
        for(i = 0; i < n; i++)
            scanf("%d %d",&p[i].x,&p[i].y);
        Graham(p,n,ch,num);
        double result = acos(-1.0)*l*2;
        for(i = 0; i < num-1; i++)
            result += dis(ch[i],ch[i+1]);
        result += dis(ch[0],ch[num-1]);
        printf("%.0f\n",result);
    }
    return 0;
}