Submission #3429937

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Source Code Expand

#include <bits/stdc++.h>

using namespace std;
#define pb push_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define rep(i,n) for(int i=0;i<(n);i++)
constexpr int MOD = 1000000007;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
constexpr int dx[] = {1, 0, -1, 0, 1, 1, -1, -1};
constexpr int dy[] = {0, -1, 0, 1, 1, -1, -1, 1};

template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec){os << "["; for (const auto &v : vec) {os << v << ","; } os << "]"; return os; }
template <typename T, typename U> ostream &operator<<(ostream &os, const pair<T, U> &p){os << "(" << p.first << ", " << p.second << ")"; return os;}

void solve() {
    int N;
    cin >> N;
    double sum = 0;
    vector<double> R(N);
    for (int i = 0; i < N; i++) {
        cin >> R[i];
    }
    sort(R.rbegin(), R.rend());
    double pi = acos(-1);
    for (int i = 0; i < N; i++) {
        if (i % 2 == 0) {
            sum += R[i] * R[i] * pi;
        } else {
            sum -= R[i] * R[i] * pi;
        }
    }
    cout << sum << endl;
}

int main() {
    std::cin.tie(0);
    std::ios::sync_with_stdio(false);
    cout.setf(ios::fixed);
    cout.precision(16);
    solve();
    return 0;
}

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