#include <bits/stdc++.h>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;
typedef unsigned long long ull;


int main(){
  int n;
  cin >> n;
  double ret = 0;
  rep(i,n){
    double x, a;
    cin >> x >> a;

    ret += a * acos(0) * 2.0;
  }

  printf("%.10lf\n", ret);
  
  return 0;
}