#include <iostream>
#include <vector>
#include <string>
#include <cmath>
#include <map>
#include <algorithm>
#include <iomanip>
#include <queue>
#include <utility>
#include <set>
#include <unordered_set>
#include <stack>
#include <iterator>
#include <random>
#include <fstream>
#include <chrono>
using namespace std;
#define int long long
void fast(){
    ios_base:: sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
}
int32_t main()
{
    fast();
    double n;
    cin >> n;
    cout << 6.28318530717958623200*n << "\n";
    return 0;
}
