#include <bits/stdc++.h>
using namespace std;
template <class T>
inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
void vout(vector<long long> &v) {
  for (long long i = 0; i < (v).size(); i++) cout << v[i] << " ";
  cout << "\n";
}
void vout(vector<string> &v) {
  for (long long i = 0; i < (v).size(); i++) cout << v[i] << " ";
  cout << "\n";
}
void vvout(vector<vector<long long> > &v) {
  for (long long i = 0; i < (v).size(); i++) vout(v[i]);
}
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
long long lcm(long long a, long long b) { return a / gcd(a, b) * b; }
long long mypow(long long x, long long n, long long m) {
  if (n == 0) return 1;
  if (n % 2 == 0)
    return mypow(x * x % m, n / 2, m);
  else
    return x * mypow(x, n - 1, m) % m;
}
long long digit(long long k, long long i) {
  string s = to_string(k);
  return s[s.size() - i] - '0';
}
void uniq(vector<long long> &v) {
  sort(v.begin(), v.end());
  v.erase(unique(v.begin(), v.end()), v.end());
}
long long ctoi(char c) {
  if (c >= '0' && c <= '9') {
    return c - '0';
  }
  return 0;
}
long long leng(long long n) {
  string s = to_string(n);
  return s.size();
}
long long digitsum(long long n) {
  long long ret = 0;
  while (n > 0) {
    ret += n % 10;
    n /= 10;
  }
  return ret;
}
long long lesscount(long long x, vector<long long> &a) {
  return lower_bound(a.begin(), a.end(), x) - a.begin();
}
long long orlesscount(long long x, vector<long long> &a) {
  return upper_bound(a.begin(), a.end(), x) - a.begin();
}
long long morecount(long long x, vector<long long> &a) {
  return a.size() - orlesscount(x, a);
}
long long ormorecount(long long x, vector<long long> &a) {
  return a.size() - lesscount(x, a);
}
long long iscount(long long x, vector<long long> &a) {
  return upper_bound((a).begin(), (a).end(), x) -
         lower_bound((a).begin(), (a).end(), x);
}
vector<long long> accum(vector<long long> &v) {
  vector<long long> ret((v).size() + 1);
  for (long long i = 0; i < (v).size(); ++i) ret[i + 1] = ret[i] + v[i];
  return ret;
}
bool comp(pair<long long, long long> a, pair<long long, long long> b) {
  if (a.second != b.second)
    return a.second < b.second;
  else
    return a.first < b.first;
}
signed main() {
  cin.tie(0);
  cout.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed << setprecision(15);
  double r;
  cin >> r;
  cout << r * acos(-1.0) * 2 << "\n";
  return 0;
}
