Решение задачи Стена с Меньшиков

#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <cmath>
using namespace std;
 
typedef pair<int, int> point;
#define X first
#define Y second
 
const double pi = 2*acos(0.0);
int n, R;
vector<point> points;
 
void input() {
    cin>>n>>R;
    points.resize(n);
    int x,y;
    for (int i=0;i<n;i++) {
        cin>>x>>y;
        points[i] = point(x,y);
    }   
}
bool cw(const point &a, const point &b, const point &c) {
    return a.X * (b.Y - c.Y) + b.X * (c.Y - a.Y) + c.X * (a.Y - b.Y) < 0;
}
vector<point> convexHull(vector<point> &p) {
 
    int k = 0;
    sort(p.begin(), p.end());
    vector<point> q(n*2); // ?
    for (int i=0; i<n; q[k++] = p[i++])
        for ( ; k>=2 && !cw(q[k-2],q[k-1],p[i]); --k)
            ;
    for (int i=n-2, t=k; i>=0; q[k++] = p[i--])
        for ( ; k > t && !cw(q[k-2],q[k-1],p[i]); --k)
            ;
    q.resize(k-1 - (q[0] == q[1]));
    return q;
}
double dist(point &a, point &b) {
    return sqrt(0.0 + (a.X - b.X)*(a.X - b.X) + (a.Y - b.Y)*(a.Y - b.Y));
}
void solve() {
    vector<point> hull = convexHull(points);
    double res = 2*pi*R;
    for (int i=0; i<hull.size();++i) {
        res += dist(hull[i], hull[(i+1)%hull.size()]);
    }
    printf("%0.0f",res);
}
int main() {
    freopen("input.txt","r",stdin);
    freopen("output.txt","w",stdout);
    input();
    solve();
}