#include <bits/stdc++.h>
using namespace std;
#define pii pair <int, int>
#define ll long long
#define pdd pair <double, double>
#define pll pair <ll, ll>
#define pb push_back
#define vi vector <int>
#define vvi vector <vi>
#define PI 3.14159
int main(){
    ios::sync_with_stdio(0);cin.tie(nullptr);
    double a;
    cin>>a;
    cout<<fixed<<setprecision(3)<<(PI*2*a)<<endl;
}