#include <bits/stdc++.h>
using namespace std;
long long int dx[4] = {-1, 1, 0, 0};
long long int dy[4] = {0, 0, -1, 1};
int32_t main(void) {
  long long int r;
  cin >> r;
  r *= 2;
  cout << (double)(r) * (double)(3.14) << "\n";
  return 0;
}
