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| #include <iostream>
#include <algorithm>
#include <cmath>
using namespace std;
const double PI = acos(-1);
const int N = 101;
int n;
int r[N];
int main() {
scanf("%d", &n);
for (int i = 0; i < n; ++i) scanf("%d", &r[i]);
sort(r, r + n, greater<int>());
double ans = 0;
for (int i = 0; i < n; i += 2)
ans += PI * (r[i] * r[i] - r[i + 1] * r[i + 1]);
printf("%lf\n", ans);
return 0;
}
|