#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <random>
#include <cassert>
#include <fstream>
#include <utility>
#include <functional>
#define popcount __builtin_popcount
using namespace std;
typedef long long int ll;
typedef pair<int, int> P;
const double PI=acos(-1.0);
int main()
{
	int n; cin>>n;
	double ans=0, a, x;
	for(int i=0; i<n; i++){
		cin>>x>>a; ans+=a;
	}
	ans*=PI;
	printf("%.10lf\n", ans);
	return 0;
}