#include <iostream>
#include <string>
#include <algorithm>
#include <cmath>
#include <deque>
#include <queue>
#include <stack>
#include <vector>
#include <utility>

#define REP(i,n) (int i = 0; i < (n); i++)
#define FOR(i,a,b) (int i=(a); i < (b); i++)
#define INF 100000000000
using namespace std;
using ll = long long;
using P = pair<int, int>;

int main()
{
    double n;
    cin >> n;

    cout << n * 2 * 3.14 << endl;
    return 0;
}