#include <iostream>
#include <string>
#include <stdlib.h>
#include <algorithm>
#include <math.h>
using namespace std;

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