#include <algorithm>
#include <cmath>
#include <vector>
#include <functional>
#include <cstdlib>
#include <map>
#include <set>
#include <iostream>
#include <string>
#include <ctype.h>
using namespace std;
#define REP(i, n) for(int i = 0; i < (int)(n); i++)
typedef long long lint;

int main(){
   int r;
   cin >> r;
   
   cout << 2*r*3.14156535 << endl;
   
   return 0;
}