#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
using ll = long long;
using P = pair<int, int>;

double r, ans;
double pi = 3.1415926535;

int main() {
  cin >> r;
  ans = pi * r * 2;
  cout << fixed << ans << endl;
  return 0;
}
