#define _CRT_SECURE_NO_WARNINGS
#include<bits/stdc++.h>
#define ll long long
#define Test int t;cin>>t;while(t--)
using namespace std;
const int dx[]={-1,-1,-1,0,0,0,1,1,1};
const int dy[]={-1,0,1,-1,0,1,-1,0,1};
void fast(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
}
int a[200001];
int main()
{
    fast();
    double r;
    cin>>r;
    const double PI=3.1415926535897931;
    cout<<fixed<<setprecision(9)<<PI*2*r;
    return 0;
}
