#include <bits/stdc++.h>
#pragma warning(disable : 4996)
const long long MOD = 1000000007;
using namespace std;
void solve() {
  int r;
  scanf("%d", &r);
  double ans = (double)r * 2 * 3.141592653589;
  printf("%.10lf\n", ans);
  return;
}
int main(int argc, char* argv[]) {
  solve();
  return 0;
}
