#include <cstring>
#include <cmath>
#include <algorithm>
#include <string>
#include <iostream>
#include <vector>
#include <queue>
#include <unordered_map>
#include <random>
#include <stack>
#include <set>
#include <unordered_set>

#define bug(x) cout<<"zdongdebug1: "<<x<<endl;
#define bug2(x, y) cout<<"zdongdebug2: "<<x<<" "<<y<<endl;
#define bug3(x, y, z) cout<<"zdongdebug3: "<<x<<" "<<y<<" "<<z<<endl;
using namespace std;
typedef long long ll;
const int maxn = 100005;
const int mod = 1000000007;


int main() {
#ifdef suiyuan2009
  freopen("/Users/suiyuan2009/CLionProjects/icpc/input.txt", "r", stdin);
  //freopen("/Users/suiyuan2009/CLionProjects/icpc/output.txt", "w", stdout);
#endif
  double  r;
  cin>>r;
  double ret = 2 * acos(-1) * r;
  printf("%.10f\n", ret);
  return 0;
}