#include <bits/stdc++.h>
using namespace std;
#define rep(i,l,r) for(int i = l; i < (r); i++)
#define pb push_back
#define ALL(a) a.begin(), a.end()
#define fastio ios::sync_with_stdio(0);cin.tie(0)
using ll = long long;
using ull = unsigned long long;

int main() {
  fastio;
  int R; cin >> R;
  cout << 2*atan(1)*4*R << endl
}
