#include<bits/stdc++.h>
const double PI=3.14159265358979323846;
using namespace std;
typedef long long ll;
#define rep(i, n) for(int i = 0; i < (n); i++)
#define rep1(i, n) for(int i = 1; i <= (n); i++)
#define co(x) cout << (x) << "\n"
#define cosp(x) cout << (x) << " "
int main(void){
    double r;cin>>r;
    co(2*r*PI);
}

