#include <bits/stdc++.h>
using namespace std;
const long double pi = acos(-1);
const long long inf = 1e18, M = 1e9 + 7;
const long long int N = 1;
void solve() {
  long double r;
  cin >> r;
  long double ans = 2 * pi * r;
  cout << fixed << setprecision(10) << ans;
}
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  long long int t = 1;
  for (long long int z = 1; z <= t; ++z) {
    solve();
  }
  return 0;
}
