#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0;i<(int)(n);++i)
#define ll long long
#define P pair<int,int>

int main(){
    const double pi=3.14159265;
    int r;
    cin>>r;
    cout<<setprecision(10)<<2*r*pi<<endl;
    return 0;
}