#include <bits//stdc++.h>
#define _USE_MATH_DEFINES
#include <math.h>
using namespace std;

int main() {
    unsigned int R;
    cin >> R;
    if (R >= 1 and R <= 100) {
        long double ans = 2.0*M_PI*R;
        cout << ans << endl;
    }
    return 0;