#include <bits/stdc++.h>
using namespace std;
int dx4[4] = {1, 0, -1, 0};
int dy4[4] = {0, -1, 0, 1};
int dx8[8] = {1, 0, -1, 1, -1, 1, 0, -1};
int dy8[8] = {1, 1, 1, 0, 0, -1, -1, -1};
int dx9[9] = {0, 1, 0, -1, 1, -1, 1, 0, -1};
int dy9[9] = {0, 1, 1, 1, 0, 0, -1, -1, -1};
const int INF = 1e9;
const long long LLINF = 1e18;
const int MAX_V = 1e6 + 1;
const long long mod = 1000000007;
double n;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cin >> n;
  cout << fixed << setprecision(2) << (n * 2.0 * 3.14159) << endl;
}
