#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using namespace std;
const double pi = 2 * acos(0.0);
int main() {
  long long r;
  cin >> r;
  cout << fixed << setprecision(10) << 2 * pi * r << '\n';
}
