include<bits/stdc++.h>
using namespace std;
#define ll long long int
#define set_bits(a) __builtin_popcount(a)
#define pb push_back
#define pf push_front
#define mod 1000000007
#define M 998244353
#define fi first
#define se second
#define endl '\n'
#define INF 1e18
#define fast ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
# define    PI                  3.14159265358979323846
 
int main()
{
    ll r;
    cin>>r;
 
    double ans = 2*PI*r;
    cout<<fixed<<setprecision(20)<<ans<<endl;
}