#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
#define db(x) cerr<<__LINE__<<": "<<#x<<" = "<<(x)<<'\n'
const int N = 1e6;
double pi = acos(-1.0);

int main() {
    ios_base::sync_with_stdio(0);
    int a,b,r,T,cas=0;
    cin>>r;
    cout<<pi*r*2;
    return 0;
}