#include <bits/stdc++.h>
using namespace std;
using vi = vector<int>;
using vii = vector<vi>;
using vs = vector<string>;
using in6 = int64_t;
using ind = double;
using pii = pair<int,int>;
using psi = pair<string,int>;
using vpi = vector<pii>;
using vps = vector<psi>;
#define rep(i, n) for (in6 i = 0; i < (in6)(n); i++)
#define repa(i,a,n) for(in6 i = a; i < (in6)(n); i++)
#define repdown(i,n) for(in6 i = n; i > 0; i--)
#define kyun ios::sync_with_stdio(false)
#define hinata cout << "\n"
#define all(c) (c).begin(), (c).end()
#define pb push_back
#define inf 2147483647
#define dset(n) cout << fixed << setprecision(n)
#define PI 3.14159265359

void yes() {
  cout << "Yes";
}

void no() {
  cout << "No";
}

int main() {
  kyun;
  int a;
  cin >> a;
  cout << 2*a*PI;
hinata;
}