#include <algorithm>
#include <climits>
#include <complex>
#include <cstring>
#include <iostream>
#include <map>
#include <set>
#include <vector>
// using
using namespace std;
// typedef
typedef long long ll;
// define
#define ALL(c) (c).begin(), (c).end()
#define SORT(c) sort(ALL(c))
#define REVERSE(c) reverse(ALL(c))
#define UNIQ(c) unique(ALL(c))
#define FOR(i, a, b, c) for (int i = (int)(a); i < (int)(b); i += (int)(c))
#define REP(i, n) FOR(i, 0, n, 1)
#define RFOR(i, a, b, c) for (int i = (int)(a); i >= (int)(b); i -= (int)(c))
#define RREP(i, n) RFOR(i, n, 0, 1)
// functions
int ceil(int a, int b) {
return (a + b - 1) / b;
}
int round(int a, int b) {
return (a + b / 2) / b;
}
int gcd(int a, int b) {
return b ? gcd(b, a % b) : a;
}
int lcm(int a, int b) {
return a * b / gcd(a, b);
}
template <class T> bool amax(T &a, const T &b) {
if (a < b) {
a = b;
return true;
}
return false;
}
template <class T> bool amin(T &a, const T &b) {
if (b < a) {
a = b;
return true;
}
return false;
}
// main
int main() {
int N, R[1000];
cin >> N;
REP(i, N) {
cin >> R[i];
}
sort(R, R + N);
reverse(R, R + N);
float res = 0;
REP(i, N) {
if (i % 2 == 0) {
res += R[i] * R[i];
} else {
res -= R[i] * R[i];
}
}
printf("%.7f\n", res * 3.14159265);
return 0;
}