#include<iostream>
#include<algorithm>
#include<string>
#include<cstdlib>
#include<vector>
#include<queue>
#include<cmath>
using namespace std;

int main()
{
    int r;
    
    cin >> r;
    
    cout << double(2 * 3.14159 * r) << endl;
    
    return 0;
}
