#include <bits/stdc++.h>
using namespace std;
const long long int mod = 1e9 + 7;
void solve() {
  double r;
  cin >> r;
  cout << fixed << setprecision(5) << 6.28318530717 * r << endl;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int total = 1;
  for (int testcase = 1; testcase <= total; testcase++) {
    solve();
  }
}
