#include <bits/stdc++.h>
using ll = long long;
constexpr int INF = 1073741823;
constexpr int MINF = -1073741823;
constexpr ll LINF = ll(4661686018427387903);
constexpr ll MOD = 1000000007;
constexpr long double eps = 1e-9;
const long double PI = acosl(-1.0L);
using namespace std;
void scans(string& str) {
  char c;
  str = "";
  scanf("%c", &c);
  if (c == '\n') scanf("%c", &c);
  while (c != '\n' && c != -1 && c != ' ') {
    str += c;
    scanf("%c", &c);
  }
}
void scanc(char& str) {
  char c;
  scanf("%c", &c);
  if (c == -1) return;
  while (c == '\n') {
    scanf("%c", &c);
  }
  str = c;
}
double acot(double x) { return PI / 2 - atan(x); }
ll LSB(ll n) { return (n & (-n)); }
template <int mod>
struct modint {
  int num;
};
int main() {
  modint<(int)1e9 + 7> unchi;
  scanf("%d", &unchi);
  printf("%.50lf\n", (double)unchi.num * 2.0 * PI);
  return 0;
}
