Submission #1305246

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Source Code Expand

#include<iostream>
#include<string>
#include<algorithm>
#include<vector>
#include<iomanip>
#define _USE_MATH_DEFINES
#include<cmath>
using namespace std;
int main(){
  int n;
  cin >> n;
  vector<int> r(n);
  for(int i=0;i<n;i++) cin >> r[i];
  sort(r.begin(),r.end());
  double sum=0;
  if(n%2==0){
    for(int i=n-1;i>=0;i--){
      if(i%2==1)sum+=r[i]*r[i];
      else sum-=r[i]*r[i];
    }
  }
  else{
    for(int i=n-1;i>=0;i--){
      if(i%2==0) sum+=r[i]*r[i];
      else sum-=r[i]*r[i];
    }
  }
  cout << setprecision(20)<< (long double)sum*M_PI << endl;
}

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