#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <string>
#include <cmath>
#include <set>
#include <bitset>
#include <limits>

using namespace std;

#define ll long long

int main(void){
    double r;
    cin >> r;

    printf("%.9lf", 2*M_PI*r);

    return 0;
}