#include <bits/stdc++.h>
using namespace std;
constexpr auto INF = 1000000000;
constexpr auto mod = 1000000007;
long long int gcd(long long int a, long long int b) {
  if (b > a) swap(a, b);
  if (a % b == 0) return b;
  return gcd(b, a % b);
}
long long int Tow(int n) {
  if (n == 0) return 1;
  long long int x = Tow(n / 2);
  x %= mod;
  x *= x;
  x %= mod;
  if (n % 2 == 1) x *= 2;
  x %= mod;
  return x;
}
int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};
char a[501][501];
bool seen[1000][1000] = {false};
void dfs(int x, int y, int h, int w) {
  seen[x][y] = true;
  for (int i = 0; i < 4; i++) {
    int Nx = x + dx[i];
    int Ny = y + dy[i];
    if (Nx >= h || Ny >= w || seen[Nx][Ny] || a[Nx][Ny] == '#' || Nx < 0 ||
        Ny < 0)
      continue;
    dfs(Nx, Ny, h, w);
  }
}
int main() {
  double a;
  cin >> a;
  double ans = 3.14159265 * a * 2;
  cout << fixed << setprecision(15) << ans << endl;
  return 0;
}
