#include<iostream>
#include<vector>
#include<cstdio>
#include<string>
#include<algorithm>
#include<map>
#include<cmath>
#include<deque>
#include<set>
using namespace std;

using ll = long long;
const int INF = 1001001001;

#define rep(i,n) for (int i = 0; i < (n); ++i)

int main(void)
{
  int r;
  cin >> r;
  printf("%.4f\n", 2 * r * M_PI);
}

