#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9 + 10;
const long long int LINF = 1LL * INF * INF;
const int MAXN = 2e5 + 10;
const int MAXM = 5e3 + 10;
priority_queue<int> pq;
vector<vector<int> > graph;
queue<int> que;
int A[MAXN];
char S[MAXN];
int main() {
  int n, m, k, a, b, x, y, q;
  int sum = 0;
  int cnt = 0;
  int mx = 0;
  int mn = INF;
  int cur = 0, idx = -1;
  int tc;
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cin >> n;
  double res = (double)n * acos(-1);
  res *= 2.0;
  cout << fixed;
  cout.precision(9);
  cout << res << "\n";
  return 0;
}
