#include <bits/stdc++.h>
using namespace std;
string vow = "aeiou";
int month[] = {-1, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
const int dxhorse[] = {-2, -2, -1, -1, 1, 1, 2, 2};
const int dyhorse[] = {1, -1, 2, -2, 2, -2, 1, -1};
const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, 1, 0, -1};
const long double pie = 3.1415926535897932384626;
const long long mod = 1e9 + 7;
void solve(int test_case) {
  long double r;
  cin >> r;
  long double ans = 2 * pie * r;
  printf("%.10Lf", ans);
  ;
}
int main() {
  int t = 1;
  for (int i = 1; i <= t; i++) solve(i);
  return 0;
}
