Submission #3771714

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Source Code Expand

#include<iostream>
#include<vector>
#include<algorithm>
#include<cmath>
#include<utility>
#include<stack>
#include<queue>
#include<map>
#include<set>

using namespace std;

#define FOR(i, a, n) for(int i = (int)(a); i < (int)(n); ++i)
#define REP(i, n) FOR(i, 0, n)

#define PRINT(mpr) printf(#mpr "\n")
#define yOUT PRINT(Yes)
#define nOUT PRINT(No)
#define YOUT PRINT(YES)
#define NOUT PRINT(NO)
 
#define pb(a) push_back(a)
#define all(x) (x).begin(),(x).end()

int main(){
    int n;
    cin >> n;
    double sum = 0, pi = 3.14159265358979323846;
    vector<double> a(n);
    REP(i, n) cin >> a[i];
    sort(all(a));

    REP(i, n){
        if(i % 2 == 0){
            sum += a[i]*a[i];
        }
        else{
            sum -= a[i]*a[i];
        }
    }

    printf("%f\n", sum*pi);
}

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