#include <iostream>
#include <utility>
#include <vector>
#include <queue>
#include <stack>
#include <array>
#include <algorithm>
#include <numeric>
#include <limits>
#include <string>
#include <regex>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;
using ll = long long;

int main()
{
    double r;
    cin >> r;
    cout << r * 6.28318530717958623200 << endl;
    return 0;
}
