#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("Ofast")
const long long MOD = (int)1e9 + 7;
const long long INF = MOD * MOD;
const long long powll = 1LL << 1;
int main() {
  double a;
  cin >> a;
  cout << M_PI * a * 2.00 << "\n";
}
