#include <iostream>
#include <string>
#include <sstream>
#include <utility>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <queue>
#include <cmath>
#include <functional>
#include <chrono>
#include <cstring>

using namespace std;
typedef long long lint;
typedef vector<int> vi;
typedef pair<int,int> pii;

typedef vector<lint> vl;
typedef pair<lint,lint> pll;

#define mpr make_pair



int main()
{
    ios_base::sync_with_stdio(0);    cin.tie(NULL);    cout.tie(NULL);

    double r; cin>>r;
    cout<<2*M_PI*r;

}
