
#include <iostream>
#include <algorithm>
#include <deque>
#include <iterator>
#include <list>
#include <map>
#include <memory>
#include <queue>
#include <set>
#include <stack>
#include <utility>
#include <vector>
#include <string>
#include <iostream>
using namespace std;
using Graph = vector<vector<int>>;
using ll = long long;
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define all(v) v.begin(), v.end()

int main() {
    int a;
    double b;
    cin >> a;
    b = a * 2 * 3.1415926;
    cout << b << endl;
}

  