#include"bits/stdc++.h"
using namespace std;
#define rep(i,n) for(int i=0;i<n;i++)
typedef long long INT;
double PI = 3.14159265;


const long long INF = 1LL << 60;

int main() {
    double r;

    cin >> r;
    
    cout << 2.0*PI*r<< endl;

    return 0;
}