#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
long long int INF = 1001001001001001LL;
int inf = 1000000007;
long long int MOD = 1000000007LL;
double PI = 3.1415926535897932;
template <typename T1, typename T2>
inline void chmin(T1 &a, const T2 &b) {
  if (a > b) a = b;
}
template <typename T1, typename T2>
inline void chmax(T1 &a, const T2 &b) {
  if (a < b) a = b;
}
int main() {
  double r;
  cin >> r;
  printf("%.10f\n", 2.0 * r * PI);
  return 0;
}
