#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1e9 + 7;
const long long INF = INT_MAX;
const long long MAX_N = 1e6 + 7;
void solve() {
  int r;
  cin >> r;
  cout << fixed << setprecision(20) << 2 * 3.141592653589793238 * r << "\n";
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  solve();
}
