#include <bits/stdc++.h>
using namespace std;
map<long long, long long> mp1, mp;
vector<pair<long long, pair<long long, long long> > > vpp, vpp1;
vector<pair<long long, long long> > vp, fin;
vector<long long> v, v1, v2, dummy;
map<pair<long long, long long>, bool> mbb;
map<long long, bool> mb;
long long n, k, a, b, c, d, ta, sum = 0, cnt = 0, m, l, req, sz, maxi;
bool sortbysec(const pair<pair<long long, long long>, long long> &a,
               const pair<pair<long long, long long>, long long> &b) {
  return (a.second < b.second);
}
vector<long long> power;
int main() {
  long long q;
  long long x;
  cin >> n;
  double ans = 0.0;
  ans = 2.00 * 3.14159 * n;
  cout << ans << endl;
  return 0;
}
