#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pll = pair<ll, ll>;
using vl = vector<ll>;
const int ARRAY = 100005;
const int INF = 1001001001;
const ll LINF = 1001001001001001001;
const int MOD = 1e9 + 7;
ll N = 0;
ll t;
ll ret = 0;
ll A = 0, B = 0, C = 0;
string s;
void input() { cin >> N; }
void solve() {
  double l = ((double)N) * 3.141592;
  cout << setprecision(10) << l << endl;
}
int main(void) {
  input();
  solve();
}
