#include<iostream>
#include<algorithm>
#include<string>
#include<vector>
#include<cstdlib>
#include<queue>

using namespace std;

#define rep(i, n) for(int i = 0; i < n; i++)
#define P pair<int, int>

int main(){
    double r;
    cin >> r;
    const double PI=3.14159;
    cout << 2 * r * PI << endl;
}
