#include <bits/stdc++.h>   using namespace std;   bool cmp(int x, int y) { return x > y; }   int main() {     double sum;     int n, r[110];     while (~scanf( %d , &n)) {       sum = 0;       for (int i = 0; i < n; i++) scanf( %d , &r[i]);       sort(r, r + n, cmp);       int tmp = 1;       for (int i = 0; i < n; i++) {         sum += tmp * r[i] * r[i];         tmp *= -1;       }       printf( %.15f n , sum * 3.141592653589793);     }     return 0;   }   