#include <bits/stdc++.h>
using namespace std;
template <typename T>
void print(T x) {
  std::cout << x << " ";
}
template <typename T>
void println(T x) {
  std::cout << x << "\n";
}
long long dp[101][101][101];
signed main(void) {
  long long R;
  cin >> R;
  std::cout << std::fixed;
  std::cout << std::setprecision(5) << R * 2 * 3.14159265359;
}
