#include "utils.cpp"

#include <bits/stdc++.h>


using namespace std;
using ll = int64_t;
#define int int64_t

 
/* ==== declarations ====== */
// functions
void solve();
void getInput();
 
// global variables
int W, H;
double pi = 3.1415926535;
/* ======================== */
void getInput() {
    
}
 
 
void solve() {
    int R;
    cin>>R;
    cout<<2*R*pi<<endl;
}
 
 
 
signed main(){
    getInput();
    solve();
    return 0;
}