import math
n = int(input())
lst = list(map(int, input().split()))
lst.sort()
ans=0
for i in range(n):
ans += (lst[i] * lst[i] * (1 - i%2 * 2))
z = round(math.asin(1) * 2 * ans, 10)
if z<0:
print(z*-1)
else:
print(z)#include <bits/stdc++.h>
using namespace std;
int main()
{
int n;
cin>>n;
int a[n+1] ={0};
for (int i = 0; i < n; i++)
{
cin>>a[i];
}
sort(a,a+n);
reverse(a,a+n);
double sum = 0;
for (int i = 0; i <=n; i+=2)
{
if(a[i] == 0 )
{
break;
}
sum+=(a[i]*a[i]-a[i+1]*a[i+1]);
}
//cout<<sum<<endl;
cout<<(3.1415926536*sum)<<endl;
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