#include<iostream>
#include<string>
#include<algorithm>
#include<vector>
#include<set>
#include<queue>
#include<cmath>
#include<map>
#include<stack>
#define ll long long
#define rep(i,n) for (int i = 0; i < (n); ++i)
using namespace std;
static const int MAXN = 100000;
static const int MAX = 1000000000;

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

    return 0;
}