#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define fi first
#define se second

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);cout.tie(NULL);
    cout<<fixed<<setprecision(5);
    double r;
    cin>>r;
    cout<<2*3.14*r;
    return 0;
}