#include <bits/stdc++.h>
using namespace std;
const long long INF = 1e9;
const long long INFLL = 1e18;
const long long MOD = 1000000007;
const long double eps = 1e-6;
const long double eps2 = 1e-9;
const long long MOD2 = 998244353;
const long long dosz = 5e5;
const long long SZ = (1 << 18);
const long double PI = atan2l(0, -1);
void fast_io() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
}
void solve() {
  long double r;
  cin >> r;
  cout << 2 * r * PI << endl;
}
signed main() {
  fast_io();
  long long q = 1;
  cout << fixed << setprecision(6);
  while (q--) {
    solve();
  }
  return 0;
}
 
