#include <bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (n); ++i)
using namespace std;
using ll = long long;
using P = pair<int,int>;

const double PI=3.14159265358979323846;

int main() {

    double r;
    cin >> r;
    double ans;
    ans = 2 * PI * r;
    printf("%.10f", ans);
    return 0;
}