#include<bits/stdc++.h>
using namespace std;
#define int long long
const double pi=3.14159265358979323846;
const int N = 10001;
int a[N];
bool cmp(int a,int b)
{
    return a>b;
}
signed main()
{
    double cnt=0;
    int n;
    cin>>n;
    for (int i = 0; i < n; i ++ )
    cin>>a[i];
    a[n]=0;
    sort(a,a+n,cmp);
    for (int i = 0; i < n; i +=2 )
    {
        cnt+=(double)(((a[i]*a[i]-a[i+1]*a[i+1])*pi));
    }
    printf("%.6f",cnt);
}
//神魔牛马QWQ
小数点后7位都过不了