#include <bits/stdc++.h>
using namespace std;
void __print(int x) { cerr << x; }
void __print(long x) { cerr << x; }
void __print(long long x) { cerr << x; }
void __print(unsigned x) { cerr << x; }
void __print(unsigned long x) { cerr << x; }
void __print(unsigned long long x) { cerr << x; }
void __print(float x) { cerr << x; }
void __print(double x) { cerr << x; }
void __print(long double x) { cerr << x; }
void __print(char x) { cerr << '\'' << x << '\''; }
void __print(const char *x) { cerr << '\"' << x << '\"'; }
void __print(const string &x) { cerr << '\"' << x << '\"'; }
void __print(bool x) { cerr << (x ? "true" : "false"); }
template <typename T, typename V>
void __print(const pair<T, V> &x) {
  cerr << '{';
  __print(x.first);
  cerr << ',';
  __print(x.second);
  cerr << '}';
}
template <typename T>
void __print(const T &x) {
  int f = 0;
  cerr << '{';
  for (auto &i : x) cerr << (f++ ? "," : ""), __print(i);
  cerr << "}";
}
void _print() { cerr << "]\n"; }
template <typename T, typename... V>
void _print(T t, V... v) {
  __print(t);
  if (sizeof...(v)) cerr << ", ";
  _print(v...);
}
void c_p_c() {}
void parray(int a[], int n) {
  for (int i = 0; i < n; i++) {
    cout << a[i] << " ";
  }
  cout << "\n";
}
int no = 1, n1;
int recu(int n, int k, int val) {
  if (n == n1) {
    return min(val * 2, val + k);
  }
  return min(recu(n + 1, k, 2 * val), recu(n + 1, k, val + k));
}
int digit(int n) {
  int temp = 0;
  while (n) {
    n = n / 10;
    temp++;
  }
  return temp;
}
bool check(int a[], int n) {
  for (int i = 0; i < n; ++i) {
    if (a[i] % 2) return false;
  }
  return true;
}
bool ispalin(int n) {
  string str = to_string(n);
  for (int i = 0; i < str.size() / 2; ++i) {
    if (str[i] != str[str.size() - i - 1]) {
      return false;
    }
  }
  return true;
}
bool is_palindrome(string s) {
  int n = s.size();
  for (int i = 0; i < n / 2; ++i) {
    if (s[i] != s[n - i - 1]) return false;
  }
  return true;
}
int check(string s, string t) {
  int sum = 0;
  for (int i = 0; i < s.size(); ++i) {
    if (s[i] == t[i]) sum++;
  }
  return sum;
}
int main() {
  int i, n, m, k;
  long long sum = 0;
  c_p_c();
  double no;
  cin >> no;
  cout << 3.14159265358979323846 * n * 2 << "\n";
}
