#include <bits/stdc++.h>
using namespace std;
#pragma GCC target("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
const long long INF = 3e18 + 12;
const int inf = 1e9;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed << setprecision(20);
  int n;
  cin >> n;
  cout << n * 2 * M_PI << endl;
  return 0;
}
