#include<bits/stdc++.h>

using namespace std;
const int N = 110;

double r[N];
int n;
double res=0;
int main()
{
    cin >> n;
    for (int i = 1; i <= n; i ++ )cin>>r[i];

    sort(r+1,r+n+1);


    for (int i = n; i >=1; i-=2)
        res+=(acos(-1)*(r[i]*r[i]-r[i-1]*r[i-1]));

    printf("%.6lf\n",res);
    return 0;
}