#include <bits/stdc++.h>

using namespace std;
using ll = long long int;
using P = pair<int, int>;

#define roop(i, n) for(int i = 0;i < n;i++)

int main(void){

    int n;
    cin >> n;
    printf("%.10lf", 2*n*acos(-1));

    return 0;
}