#include <bits/stdc++.h>
using namespace std;
int mpow(int base, int exp);
void ipgraph(int m);
void dfs(int u, int par);
const int mod = 1000000007;
const int N = 3e5, M = N;
vector<int> g[N];
int a[N];
int main() {
  ios_base::sync_with_stdio(0);
  int n;
  cin >> n;
  double pi = 3.141592653589793238;
  double ab = 2 * pi * n;
  cout << setprecision(20) << ab << endl;
  return 0;
}
