///mAAria...
#include<bits/stdc++.h>
 
using namespace std;
 
#define int long long

const long double PI = acos(-1);

const int maxn = 2*1e5 + 10;


void solve(){
    int r;
    cin >> r;

    double ans = 2.0*PI*(double)r;

    cout << fixed << setprecision(12) << ans << endl;
    

} 

signed main() 
{   
    IOS;
    int t = 1;
    // cin >> t;

    for(int i = 1;i<=t;i++){
        solve();
    }
    return 0;
} 

 
