#include <bits/stdc++.h>

#define pan(i,n) for(int i=0;i<n;i++)
#define pans(i,n,a) for(int i=a;i<n;i++)
#define ll long long int
#define vi vector<int>
#define vl vector<ll>
#define pb push_back
#define INF 1e9+7
#define PI 3.141592653
#define inf 100007
#define P pair<ll,ll>

using namespace std; 

int main(){
    int r;
    cin>>r;
    double ans=2*PI*r;
    cout<<ans;
}