#include <bits/stdc++.h>
#define rep(i,n) for (int i=0;i<(n);++i)
using namespace std;
using ll = long long;

int main() {
  int r;
  ll p;

  cin >> r;
  cout << fixed << setprecision(10);
  cout << 2*r*3.14159265358979 << endl;
  return 0;
}