TIOJ 2167 (AC)

#include <bits/stdc++.h>
#include <time.h>
using namespace std;
#define SZ(x) (int)(x.size())
#define int long long

int x[500001], y[500001];

int32_t main(){
    int n;
    cin >> n;
    for(int i = 0; i < n; ++i){
        cin >> x[i] >> y[i];
    }
    sort(x, x+n);
    sort(y, y+n);
    long double ans = 0;
    long double cnt = 0;
    long double sum = 0;
    for(int i = 1; i < n; ++i){
        int k = x[i] - x[i-1];
        cnt = cnt + 2 * k * sum + (long double)i * k * k;
        ans += cnt;
        sum += k * i;
        //cout << i << ' ' << k << ' ' << cnt << ' ' << sum << '\n';
    }
    sum = cnt = 0;
    for(int i = 1; i < n; ++i){
        int k = y[i] - y[i-1];
        cnt = cnt + 2 * k * sum + (long double)i * k * k;
        ans += cnt;
        sum += k * i;
        //cout << i << ' ' << k << ' ' << cnt << ' ' << sum << '\n';
    }
    printf("%.9LF\n", ans * acos(-1) / 2);
    if(ans * acos(-1) / 2 < 0)while(1);
    return 0;
}