#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0;i<(n);i++)
#define sz(x) int(x.size())
#define pb push_back
using ll=long long;
const ll MOD=1000000007;



int main() {
    double R;
    cin>>R;
    //小数第10位まで出力
cout<<fixed<<setprecision(10);
    cout<<R*2*3.1415926535<<endl;
}