#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>
#include <cmath>
#include <map>

#define reps(i,s,n) for(int (i) = (s); (i) < (n); (i)++)
#define rep(i,n) reps(i,0,n)
using namespace std;
using ll = long long;

int main(){
    float ans;
    float r;
    cin >> r;
    ans = 2* 3.14159263 * r;
    printf("%.6f\n",ans);

    return 0;
}