#include <algorithm>
#include <iostream>
#include <vector>
#include <string>
#include <sstream>
#include <utility>

using namespace std;

typedef pair<int,int> pii;
typedef vector<int> vi;

int main () {
    // N
    // A_1 ...... A_N
    int R;
    cin >> R;
    cout << 2*3.14*R << endl;
    return 0;
}