#include <bits/stdc++.h>
#define ll long long
#define V vector<long long>
#define VV vector<vector<long long>>
#define VVV vector<vector<vector<long long>>>
#define rep(i,n) for(ll (i)=0;(i)<(n);++(i))
using namespace std;

int main() {
    double r;
    cin>>r;
    cout<<2*r*M_PI<<endl;
}
