#define _USE_MATH_DEFINES
#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define MT make_tuple
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
#define RT return
using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;
using vll = vector<ll>;

const int MA = 1000000;
// Σ sin(x+a)/(x+a)
double test(double a) {
    double res = 0;
    for (int x = -MA; x <= MA; ++x) {
        if (abs(x + a) < 1e-8) {
            res += 1;
        } else {
            // debug(sin(x + a) / (x + a));
            res += sin(x + a) / (x + a);
        }
    }
    return res;
}

void solve() {
    int N;
    cin >> N;
    double ans = 0;
    rep(i, N) {
        double x, a;
        cin >> x >> a;
        ans += a;
    }
    ans *= M_PI;
    cout << ans << endl;
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout << fixed << setprecision(15);
	solve();
	return 0;
}