#include <bits/stdc++.h>
using namespace std;
const int INF = 0X3F3F3F3F;
const int MIN = -(1 << 30);
const long long N = 1e5 + 7;
inline void solve() {
  double n;
  cin >> n;
  printf("%.6lf\n", n * acos(-1) * 2);
}
int main() {
  solve();
  return 0;
}
