clude<bits/stdc++.h>
#define ll long long int
#define ld long double
#define fast ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
using namespace std;
 
int main(){
    fast;
    ll n;
    cin >> n;
    ld ans = 2*n*3.14;
    cout << ans;
    return 0;
}
//                                     #!!{*[$um1t_Kum@r]*}!!#