#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC optimize("no-stack-protector")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,popcnt,abm,mmx,tune=native")
#pragma GCC optimize("fast-math")
using namespace std;
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
const int MAXN = 100;
const int INF = 1e9;
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  long double r;
  cin >> r;
  cout << fixed << (long double)2 * (long double)3.14159265358979323846 * r
       << '\n';
  return 0;
}
