#include <bits/stdc++.h>
using namespace std;
#define rep(i,n)for(int i=0;i<(n);i++)
#define ALL(a) (a).begin(), (a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define pb push_back
using ll = long long;
using P = pair<int, int>;
using Graph = vector<vector<int>>;

const double PI = 3.1415926535;

int main(){
    double r; cin >> r;
    double ans = 2.0 * PI * r;
    cout << fixed << setprecision(5) <<  ans << endl;
}