#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>

#define REP(i,n) for(int i=0, i##_len=(n); i<i##_len; ++i)
#define all(x) (x).begin(),(x).end()
#define int long long
#define pb push_back
using ll = long long;
using namespace std;
int gcd(int a,int b){return b?gcd(b,a%b):a;}
void printV(std::vector<int> const &input){for (int i = 0; i < input.size(); i++)std::cout << input.at(i) << ' ';}

signed main(void){
    int R;
    cin >> R;
    long double temp=2*R*M_PI;
    cout.precision(32);
    cout << temp << endl;
    return 0;
}