#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <unordered_map>
#include <unordered_set>
#include <algorithm>
#include <cmath>

using namespace std;

using ull_t = unsigned long long;

int main()
{
    int R;
    cin >> R;
    cout << 2 * R * M_PI << endl;
}
